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post #1 of 60 Old 11-06-2012, 06:55 AM - Thread Starter
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Within the PCM specification or other digital audio standards for that matter, what is considered a logical 1 in terms of voltage level? I am curious as It may pertain to the amount of signal degradation that is acceptable for long runs of digital audio transmission. Is it 5V? 1V? Or does it vary from component to component (which seems less likely to me)?
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post #2 of 60 Old 11-06-2012, 07:37 AM
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Originally Posted by theeld View Post

Within the PCM specification or other digital audio standards for that matter, what is considered a logical 1 in terms of voltage level? I am curious as It may pertain to the amount of signal degradation that is acceptable for long runs of digital audio transmission. Is it 5V? 1V? Or does it vary from component to component (which seems less likely to me)?

The voltage associated with true or logic 1 is dependent on the equipment. Once upon a time most digital logic was TTL, but those days are gone.

http://en.wikipedia.org/wiki/Logic_level
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post #3 of 60 Old 11-06-2012, 07:55 AM - Thread Starter
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If it is dependant upon the equipment, that would seem that it would lend itself to alot of equipment not working together (like a DVD player not working with a Receiver). However unless that circumstance is more prevalant than I perceive, I believe most digital sources seem to work without issue with avr's, dac's, processors etc....
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post #4 of 60 Old 11-06-2012, 08:16 AM
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post #5 of 60 Old 11-06-2012, 08:53 AM
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Quote:
Originally Posted by theeld View Post

Within the PCM specification or other digital audio standards for that matter, what is considered a logical 1 in terms of voltage level? I am curious as It may pertain to the amount of signal degradation that is acceptable for long runs of digital audio transmission. Is it 5V? 1V? Or does it vary from component to component (which seems less likely to me)?
The specification for S/PDIF calls for 0.6 volts peak to peak with nominal value of 0.5 volts. Alas, there is no certification of it nor does anyone ever test for it. As a result, the value is all over the place in consumer gear. Here is a measurement I made for another reason with two different cables from the output of a DVD player over S/PDIF:

i-SPtN6Nd-XL.png

Eyeballing the blue trace, peak to peak is 2.4 volts or 4X of what the spec mandates. Note that the level also varies based on the cable as you mention. In this case, the second trace is a low bandwidth cable which has caused the waveform to distort and with it, also lowers the peak to peak value.

Minimum level for input is specified at 0.2 volts.

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post #6 of 60 Old 11-06-2012, 09:48 AM - Thread Starter
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Originally Posted by amirm View Post

The specification for S/PDIF calls for 0.6 volts peak to peak with nominal value of 0.5 volts. Alas, there is no certification of it nor does anyone ever test for it. As a result, the value is all over the place in consumer gear. Here is a measurement I made for another reason with two different cables from the output of a DVD player over S/PDIF:
i-SPtN6Nd-XL.png
Eyeballing the blue trace, peak to peak is 2.4 volts or 4X of what the spec mandates. Note that the level also varies based on the cable as you mention. In this case, the second trace is a low bandwidth cable which has caused the waveform to distort and with it, also lowers the peak to peak value.
Minimum level for input is specified at 0.2 volts.

Thanks for that graphic. That is exactly what I was after. As for the last statement "Minimum level for input is specified at 0.2 volts". Are you referring to the voltage at which your sound processor or DAC considers logical 1? If so, is that adjustable on the equipment you are using?
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post #7 of 60 Old 11-06-2012, 10:49 AM
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S/PIDF uses biphase mark encoding. Data is not represented by voltages, but by transitions, or the lack thereof, between voltages.
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post #8 of 60 Old 11-06-2012, 10:55 AM
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Quote:
Originally Posted by amirm View Post

The specification for S/PDIF calls for 0.6 volts peak to peak with nominal value of 0.5 volts. Alas, there is no certification of it nor does anyone ever test for it. As a result, the value is all over the place in consumer gear. Here is a measurement I made for another reason with two different cables from the output of a DVD player over S/PDIF:

i-SPtN6Nd-XL.png

Eyeballing the blue trace, peak to peak is 2.4 volts or 4X of what the spec mandates. Note that the level also varies based on the cable as you mention. In this case, the second trace is a low bandwidth cable which has caused the waveform to distort and with it, also lowers the peak to peak value.

The measurement above is probably incorrect because SP/DIF is defined to be an impedance-matched system, and I see no sign that you honored this requirement.,

By just putting a scope on the end of a cable, you did not provide the specified termination of 75 ohms. Properly designed SP/DIF inputs provide a 75 ohm termination.

Here is an example:



The correct way to do this measurement is to put a 75 ohm load across the input terminals of the scope. This will have the effect of reducing the voltage to about half of what you measured with the apparently incorrect methodology. It will also significantly improve the square wave response of the cable.
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post #9 of 60 Old 11-06-2012, 10:57 AM
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Originally Posted by Colm View Post

S/PIDF uses biphase mark encoding. Data is not represented by voltages, but by transitions, or the lack thereof, between voltages.

This is true, but SP/DIF inputs still need a certain range of voltages to operate properly. Amir's statements about voltages look about right to me, but his measurement technique seems to be incorrect and likely to obtain incorrect readings.
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post #10 of 60 Old 11-06-2012, 11:24 AM - Thread Starter
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Quote:
Originally Posted by Colm View Post

S/PIDF uses biphase mark encoding. Data is not represented by voltages, but by transitions, or the lack thereof, between voltages.

Agreed, there is a specific trigger voltage that will saturate the transistor(s) and will equate to a TRUE from a bitwise persepective. That trigger voltage I do believe will likely vary do to manufactorer/design/chipset etc so I imagine it depends on the DAC/Processor specifically as to how low a Vpp will equate to correctly transcoding a PCM signal.
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post #11 of 60 Old 11-06-2012, 11:51 AM
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Is there an input buffer stage (ie opamp, typically) on the receiving end that, for lack of a better term, "smooths out" any differences in voltage swing in an incoming SPDIF signal?
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post #12 of 60 Old 11-06-2012, 12:22 PM
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Quote:
Originally Posted by theeld View Post

Thanks for that graphic. That is exactly what I was after. As for the last statement "Minimum level for input is specified at 0.2 volts". Are you referring to the voltage at which your sound processor or DAC considers logical 1? If so, is that adjustable on the equipment you are using?
As Colm correctly states, the waveform is a few steps removed from what the DAC sees (it is both modulated and reformatted). Briefly the detection mechanism is a "zero crossing" circuit. Two transitions mean a "1" and a single means a "0." There is more to it than this and I hate to get into it any more as to risk confusing you and everyone else smile.gif. Fortunately for the purposes of your original question of reliability of capturing the (generic) bits properly, it is not important what the bits mean at this level. What matters is that we can reliably detect that we have crossed zero. To that end, the more "headroom" we have above and below zero, the easier it is to detect zero crossing in the face of noise, timing distortion (pulses jumping back and forth), and rise time of the pulse. All of these factors however contribute to reliable detection of the pulse so the amplitude of the waveform alone is only one factor.

The 0.2 volt peak to peak is what the spec says the receiver should see. That is, 0.1 volt above zero and 0.1 volt below zero. Should the swing be lower than this, the receiver may still capture the bit correctly. But the spec gives it a "get out of jail free card" should it not be able to do so.

And no, it is not adjustable. Whatever the required peak to peak value is, is a function of the receiving circuit. Different gear therefore will have different circuit designs and hence, differing ability to capture borderline signals. As I noted, this is almost never tested or published so short of testing and measuring it yourself, you have no way of knowing.

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post #13 of 60 Old 11-06-2012, 12:30 PM
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Originally Posted by JHAz View Post

Is there an input buffer stage (ie opamp, typically) on the receiving end that, for lack of a better term, "smooths out" any differences in voltage swing in an incoming SPDIF signal?
Yes. Indeed, you need to filter out the noise or the circuit will start to oscillate at zero crossing point. This also builds some useful hysteresis in the design. In addition to voltage filtering, timing also needs to be filtered to reduce jitter. The standard does not specify how any of this is done so there is a lot of freedom in building good and not so good implementations. That said, for consumer applications of a few feet of S/PDIF, they all universally work in delivering a reliable bit stream. It only becomes an issue when cable lengths become long as OP asked about. You can see how clean the Cyan waveform is for my 1 meter generic coax cable. Even the distorted low-bandwidth cable generated reliable bit stream (although pass through jitter was far higher: 5000 picoseconds vs 435 picoseconds).

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post #14 of 60 Old 11-06-2012, 01:00 PM - Thread Starter
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Thank you for all the insight presented here. It has got me thinking, Is there something other needed than just a simple DAC to transform PCM audio to a pre-amp level suitable for a power amplifier input? Or do commercially available DAC's have this "filtering stage" ingrained in their design?
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post #15 of 60 Old 11-06-2012, 03:01 PM
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Quote:
Originally Posted by theeld View Post

Thank you for all the insight presented here. It has got me thinking, Is there something other needed than just a simple DAC to transform PCM audio to a pre-amp level suitable for a power amplifier input? Or do commercially available DAC's have this "filtering stage" ingrained in their design?
You are quite welcome. Alas, it seems that our deeper dive managed to create confusion smile.gif. The filtering and such that we are talking about is for conditioning the digital stream so that it is easier to capture the digital data. It is not material to what you need to interface a DAC to a power amplifier. The (analog) output of the DAC is usually fine for amplification. The only issue is that the DAC will play at max volume so you need some way of adjusting the level to your taste that normally a pre-amp does. Solution is to get a DAC with built-in volume control. Alternatively, you can change the PCM digital values prior to it getting to the DAC. Music players/OS can do this although the quality of volume control there needs to be examined to make sure it doesn't raise distortion/lose resolution.

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post #16 of 60 Old 11-07-2012, 04:53 AM
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Quote:
Originally Posted by theeld View Post

Thank you for all the insight presented here. It has got me thinking, Is there something other needed than just a simple DAC to transform PCM audio to a pre-amp level suitable for a power amplifier input? Or do commercially available DAC's have this "filtering stage" ingrained in their design?

The canonical design for an audio DAC component is a line receiver, a DAC, an output buffer, and a power supply. Each of those functions can be accomplished by a dedicated chip,

Add a DSP and more output buffers, and then you have surround processor. Usually a microprocessor needs to be added to control the DSP.

There are parts that include the whole works minus power supply in one chip. Usually these parts are at the more modest, cost-effective end of the cost spectrum. However, the best of them do not compromise audible quality in any reliable way.

The line receiver, the output buffer and the power supply can be implemented with discrete parts or low-complexity integrated circuits without running costs up exorbitantly, but with minimal or no performance benefits. This doesn't keep people from using discrete part construction as a feature.

The DAC function itself is generally best implemented as a chip, unless the budget and performance goals for the project is fairly stratospheric.

The line receiver chip has the responsibilities that you are asking about. If you read the spec sheets, tech articles, white papers and advertising blurbs for a number of them, you might be able to pick up on the common threads, design goals, and concerns.
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post #17 of 60 Old 11-07-2012, 02:27 PM
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I have read that AES/EBU has a voltage of 2.7pp and S/PDIF 0.5pp. What component determines/creates these voltages? What would it take to make S/PDIF 2.7pp? Thanks.
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post #18 of 60 Old 11-07-2012, 02:59 PM
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That would depend on the driver circuitry. With some transmitter ICs it could be as simple as changing/adding resistors and a cut or jump.
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post #19 of 60 Old 11-08-2012, 08:54 PM
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Quote:
Originally Posted by GGA View Post

I have read that AES/EBU has a voltage of 2.7pp and S/PDIF 0.5pp. What component determines/creates these voltages? What would it take to make S/PDIF 2.7pp? Thanks.
Don't listen to Colm. He is trying to put me out of business of answering questions. biggrin.gif

Expanding on what he said, AES standard is differential and has a range of 2 to 7 volts. The implementation of both AES and SPDIF is through a driver/transciever IC (integrated circuit). Often the same device can generate AES and SPDIF depending on how its output is wired. For AES, it will have a positive and negative drive. For SPDIF, it will be ground references so half the voltage swing is available. That voltage is still much higher than 0.5 volt of SPDIF. So a voltage divider is used to both generate the right voltage and output impedance of 75 ohms. Modification of this circuit can change the output voltage and is what Colm was talking about. If you do change it, you need to still maintain 75 ohms or you will cause other problems.

AES is available on a number of DACs and PC audio cards. So if you want higher voltage swings, the best is to use AES. Modifying the output requires some electronics knowledge per above so it is not advised for end users. smile.gif

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post #20 of 60 Old 11-08-2012, 10:45 PM
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Originally Posted by amirm View Post

Don't listen to Colm. He is trying to put me out of business of answering questions.
Wouldn't think of it. You do such a good job.
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post #21 of 60 Old 11-09-2012, 10:23 AM
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Originally Posted by arnyk View Post

The correct way to do this measurement is to put a 75 ohm load across the input terminals of the scope. This will have the effect of reducing the voltage to about half of what you measured with the apparently incorrect methodology. It will also significantly improve the square wave response of the cable.

Was this ever addressed further? Arny, can I assume that a typical RCA digital input provides the proper termination?

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post #22 of 60 Old 11-09-2012, 10:30 AM
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Quote:
Originally Posted by Ethan Winer View Post

Quote:
Originally Posted by arnyk View Post

The correct way to do this measurement is to put a 75 ohm load across the input terminals of the scope. This will have the effect of reducing the voltage to about half of what you measured with the apparently incorrect methodology. It will also significantly improve the square wave response of the cable.

Was this ever addressed further?

Not that I can see. You know we got folks around here that cut and run if you catch them with their hands in the cookie jar! ;-)


Quote:
Arny, can I assume that a typical RCA digital input provides the proper termination?

For sure, as do video inputs. Take a look at the schematic of a typical SPDIF input that I posted in post #8:



Check out R13 right after the input on the left. 75 ohms!
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post #23 of 60 Old 11-09-2012, 11:25 AM
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Take a look at the schematic of a typical SPDIF input that I posted in post #8:

Where's the transformer that the specification calls for?
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post #24 of 60 Old 11-09-2012, 11:36 AM
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Originally Posted by SAM64 View Post

Quote:
Take a look at the schematic of a typical SPDIF input that I posted in post #8:

Where's the transformer that the specification calls for?

Transformers are most commonly placed on SPDIF outputs. There, they have a dual role of providing both isolation but also as EMI filters for FCC Part 15 compliance.
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post #25 of 60 Old 11-09-2012, 11:52 AM
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Doesn't Amir's AP test rig provide the proper termination? I sort of figured it would but do not know (and am too lazy/busy to look it up).

One catch with getting an AES card is that it might not play nicely with consumer S/PDIF cards. Some allow you to change the levels and drive single-ended but not all. At least IME (fairly limited, granted).

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post #26 of 60 Old 11-10-2012, 10:37 AM
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Quote:
Originally Posted by arnyk View Post

Take a look at the schematic of a typical SPDIF input that I posted in post #8 ... Check out R13 right after the input on the left. 75 ohms!

Gotcha. That makes perfect sense.
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Doesn't Amir's AP test rig provide the proper termination?

Since Amir hasn't replied, I think I can guess the answer.

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post #27 of 60 Old 11-10-2012, 12:08 PM
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Originally Posted by DonH50 View Post

Doesn't Amir's AP test rig provide the proper termination? I sort of figured it would but do not know (and am too lazy/busy to look it up).
Hi Don. I have been avoiding this point as to not confuse OP since it is unrelated to the topic at hand. But seeing how folks are reading into my lack of response, here we go smile.gif.

I think Arny confused the output that I showed with that of an oscilloscope where its default impedance is very high (1 megaohm or higher). As you correctly observed, such was not the case with my measurement. It was performed on an audio analyzer and the input I used is its unbalanced connection that is made to measure S/PDIF. In that regard, it is normally terminated to 75 ohms. The AP does however have the option to remove the termination under its more advanced setting. Indeed, I had used that option as part of this larger test which was to determine the termination impedance of the source/cable. In the graph I post here however, termination was in place at 75 ohms. So the data was as presented.

Here is the same test except this time I have overlaid the unterminated input of the AP on the terminated version for both the generic high-bandwidth cable and the low-bandwidth one:

i-6L3PQRN-XL.png

As expected, the usual effect is there. Without the termination resistor, the output voltage shoots up and there is slight amount of ringing together with some rolling off the high frequencies. You can see this in the green vs yellow (terminated vs. not respectively). Similar thing goes on with the low-bandwidth cable with its jagged response. It is just a different shape of bad smile.gif. With respect to OP's question, all four of these scenarios created a reliable transmission although the analyzer did raise a warning, lighting up its "confidence" flag for the worst case signal: unterminated low-bandwidth. So from data transmission point of view, all was well.

It was a different story with respect to quality of the timing for each sample though. Jitter which was already bad for the low-bandwidth cable at around 5,000 picoseconds, shot up to 40,000 picoseconds (no doubt the reason for above warning from AP). As a reminder, the high bandwidth generic cable was at 500 picoseconds and was unphased by impedance change.

As always when we look at digital audio transmission, we need to keep in mind that it must convey two things: (1) the digital audio samples and (2) "when" those samples need to be output. The former is very robust in consumer applications with short cables. The latter less so although in this case I could not harm a high-bandwidth cable even after screwing up the "transmission line" (within the limits of my measurement tool).

Finally, let me note that the low bandwidth cable is actually a high-end audiophile cable. It is not however designed for this application. It is an analog interconnect with a filter network. I used it here as an easy way to simulate what cable bandwidth does to the transmission and jitter without having to manually build such a circuit. So unless you have really, really long cables, you are not going to see the degraded response I am showing here. Think of it as a learning tool as you read these measurements. It is not meant at all to justify expensive S/PDIF cables. Indeed the data says you the cheapest thing worked great in this scenario (again within the limits of my measurements which is a few hundred picoseconds of jitter).

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post #28 of 60 Old 11-10-2012, 01:10 PM
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Thanks Amir, I figured. BTW, the 'scope probes I use include both 50 ohm (RF, not video, world) and high-impedance active probes (with the latter the system is terminated on-chip or on-board). The probes cost as much as the 'scopes we used to get not so long ago, and the DSO prices would buy a decent house... smile.gif

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post #29 of 60 Old 11-11-2012, 11:54 AM
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Quote:
Originally Posted by amirm View Post

The AP does however have the option to remove the termination under its more advanced setting. Indeed, I had used that option as part of this larger test which was to determine the termination impedance of the source/cable.

...

the low bandwidth cable is actually a high-end audiophile cable. It is not however designed for this application. It is an analog interconnect with a filter network.

Even with this new explanation, it's still not totally clear what you showed originally in Post #5. Are you now saying that your original "look at how bad this is" graph was not only unterminated, but also used a "special audiophile" wire that intentionally rolls off the high end? If so, is there a reason you didn't say so in that first post? Or in the other threads where you've posted that same graph?

--Ethan

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post #30 of 60 Old 11-14-2012, 08:31 AM
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