HDMI vs. S/PDIF/Coax? - AVS Forum
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post #1 of 12 Old 06-18-2007, 05:07 PM - Thread Starter
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Here's another one of my paranoid posts.

The first thing I'm going to worry about when I get a new receiver with HDMI audio processing is the quality of regular Dolby and DTS output.

I've read some things which implied that HDMI may be more prone to jitter than optical or coax because it uses fewer clocks (?). Is there anyone out there that has actually preferred the quality through your traditional digital connector, rather than HDMI?
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post #2 of 12 Old 06-18-2007, 05:15 PM
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I do, but I compared a mid level hdmi to a high end processor using a dd signal. I liked the optical better. Now playing uncompressed audio is a different story and the only reason I switched, it is much better than lossy formats. I would not get hdmi if you are planning on using it for regular dvd'd mostly. Remember I compared to a high end processor. If I compared to a lesser processor than maybe the hdmi would be just as good.
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post #3 of 12 Old 06-19-2007, 07:18 AM
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I say no worries, there is no worry of issues with this at all by me personally.
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post #4 of 12 Old 06-19-2007, 07:28 AM
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Don't worry about jitter. Leave that to the engineers that design your product. If you buy from a good brand (as opposed to some cheap brand that just doesn't care), it won't be an issue.

The people that worry about jitter are either engineers designing the products, else they're people who also tell you what speaker cables to buy, how to keep them off the floor, how to mark your CDs with a marker for better sound, etc.

"Vintage" is good for wine, not for A/V equipment.

-Dan D.
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post #5 of 12 Old 06-19-2007, 07:52 AM
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I did some googling, but I did not see anything concrete on this topic. HDMI seems to use a single clock for all three TDMS channels, and the clock denotes the "video pixel rate." The clock itself seems to use TDMS, which implies that the clock is sent over a balanced pair of wires (I am not an engineer, I am speculating based on my interpretation of the HDMI document.)

None of that is very helpful. Firstly, jitter is one of those highly debated topics. Many people think it's irrelevant with properly made cables and electronics. Other people are spooked by it, and have persued various options such as getting expensive aftermarket DACs. Secondly, I suspect few of us really understand the engineering. HDMI, in particular would have to build up some sort of audio clock from the HDMI clock - there can't be too many people who understand the ramifications of the process.

I certainly have no clue whether or not HDMI has more or less jitter, and I won't speculate on something so complex. Your best option seems to try it out both ways.

I suspect digital audio is here to stay, it will only get more complex and some people will always be suspicious of it. Those people are keeping vinyl alive, and/or creating a nice niche market of special digital audio gear such as reclocking devices.

"But this one goes up to 11"
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post #6 of 12 Old 06-19-2007, 08:06 AM
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To me atleast compressed audio(DD, DTS) sounds harsh over HDMI compared to coaxial which is much smoother & enveloping. This is on Integra DTR-7.7.

Vinod
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post #7 of 12 Old 06-19-2007, 08:15 AM
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Like I said, I like optical or coax better than HDMI for compressed audio, but I did not buy HDMI for compressed audio. Uncompressed audio is so much better.
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post #8 of 12 Old 06-19-2007, 10:04 AM
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Quote:
Originally Posted by MichaelJHuman View Post

Secondly, I suspect few of us really understand the engineering.

All the more reason for us to leave it to the real engineers.

"Vintage" is good for wine, not for A/V equipment.

-Dan D.
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post #9 of 12 Old 06-19-2007, 10:31 AM
 
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HDMI is synchronous, it carries a clock signal at the pixel rate, which is obviously much higher then the clock required for digital audio. Clock signals can be easily divided down to an appropriate rate.

With SPDIF over optical or coax, the clock signal must be derived from the digital data, via a PLL...which method do you think delivers a more stable signal.
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post #10 of 12 Old 06-19-2007, 10:45 AM
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You know what I think? I think either way works just fine.

"Vintage" is good for wine, not for A/V equipment.

-Dan D.
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post #11 of 12 Old 06-19-2007, 01:30 PM
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I think what donoman thinks, I never complained about either. You guys are looking to much into this. I started to get crazy with upgrades and the more I did it the more everything sounds alike. There are differences but there are more similarities. If you watch a movie and get that wow factor and just enjoy it than that is what matters. I get the wow factor with uncompressed audio more than ever. I hope it lasts.
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post #12 of 12 Old 06-20-2007, 01:22 PM
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Quote:
Originally Posted by MichaelJHuman View Post

I did some googling, but I did not see anything concrete on this topic. HDMI seems to use a single clock for all three TDMS channels, and the clock denotes the "video pixel rate." The clock itself seems to use TDMS, which implies that the clock is sent over a balanced pair of wires (I am not an engineer, I am speculating based on my interpretation of the HDMI document.)

None of that is very helpful. Firstly, jitter is one of those highly debated topics. Many people think it's irrelevant with properly made cables and electronics. Other people are spooked by it, and have persued various options such as getting expensive aftermarket DACs. Secondly, I suspect few of us really understand the engineering. HDMI, in particular would have to build up some sort of audio clock from the HDMI clock - there can't be too many people who understand the ramifications of the process.

I've worked with both HDMI and S/PDIF, so I'll give it try!

Comparing the jitter in HDMI and its effect on the audio information, and the jitter in S/PDIF coax / optical is a bit like comparing apples and oranges.

The HDMI clock is sent using 1 of 4 TMDS channels, the other three channels are the data channels. 10 bits of information are sent for each clock. The receiver must lock onto the clock, and using a Phase Locked Loop, create an internal clock that is 10 times faster than the clock being sent over the TMDS line and it then uses this x10 clock to capture the 3 data streams.

The audio information being sent over HDMI is sent as data packets in what they call the "Data Island Periods". These are little islands of otherwise unused time during horizontal and vertical sync times. The newer HDMI standards simply defined a protocol that made use of this unused time by injecting audio packets.

For audio to be heard, the receiver must extract these packets, and perform any error corrections that may be needed. Since these packets come in high speed bursts, the data must be buffered, if it's a compressed format, the data must be uncompressed, it must be routed to the proper channels (stereo, 5.1, 7.1), it then must be delayed (more buffering) the proper amount of time to be decoded in sync with the video, especially if any type of video scaling is being done.

So while HDMI is very susceptible to jitter, the results are sparkles, blue screens, loss of HDCP, complete loss of audio, that sort of thing.

Since HDMI audio is sent in packets (similar to streaming audio over the net), any jitter in the sample rate clock will be (as stated earlier) an engineering problem, completely independent of the HDMI clock. Just like streaming audio has nothing to do with when TCP/IP packets are received, as long as the data is there before you need it, there won't be any gaps in the audio.

S/PDIF on the other hand sends its data as a continuous stream and the clock is in sync with the sample rate. A S/PDIF decoder locks onto the data stream's clock, and as each word is received, it is to sent to a DAC and the clock from the S/PDIF stream is used to latch the data into the DAC. So if there is jitter in the S/PDIF clock, depending upon the design, there can be jitter in the latching of each word into the DAC.

So which one is better from a jitter point of view? It's hard to say, S/PDIF jitter can be affected by the source jitter, where HDMI audio jitter is independent of the HDMI clock and is dependent upon how well the audio decoder was designed.

For my ears, I'm much more concerned about compression than jitter. The loss of high frequencies and dynamic range in compressed audio (in general) drives me crazy. My CD collection is ripped in FLAC (a lossless) format. I have a Sirius sat. receiver in my truck, but I mostly listen to the comedy channels and NPR, the music channels are horrendous sounding, but I'll bet the internal jitter is as stable as a quartz crystal!

-Zonn
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