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Help, something's killing DTS passthrough...

post #1 of 14
10/25/08 at 7:17am
Thread Starter 
I recently purchased an Apogee DVI-D To HDMI converter which injects digital audio from coax or optical. See the info page here:
http://apogeeinc.net/products/item.aspx?item=2745

The video works fine, but I'm having some problems with the audio passthrough. Here is my configurational objective:
Audio output device -> Coaxial digital --> Apogee converter --> HDMI cable --> Panasonic TH-42PZ80U TV --> optical cable --> DTS receiver

This configuration works for 2 channel digital, but it does not work for DTS audio.

However, when I bypass the Apogee converter, DTS works correctly:
Audio output device -> Coaxial digital --> DTS receiver
Audio output device -> Optical digital --> DTS receiver

Literally, I can hot swap the cable between a direct link to the receiver and the Apogee converter and the audio alternatively works and fails. If I turn off DTS on the audio output device, the audio goes through the Apogee properly. If I turn it on, no audio reaches the receiver.

Anybody have experience with this converter? Or TV? I don't have any other DTS HDMI audio feeds that I can test to verify the TV passes DTS through, but I can't imagine why it wouldn't.
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post #2 of 14
10/25/08 at 7:20am
It may be the TV. Try using the Apogee and bypassing the TV.
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post #3 of 14
10/25/08 at 7:48am
Thread Starter 
I have no way to bypass the TV. The receiver only accepts optical and coaxial digital audio input.

The TV has no problem driving 5.1 from ATSC OTA HDTV broadcasts, however, so I am a bit skeptical it would be failing at DTS pass-through.
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post #4 of 14
10/25/08 at 7:55am
You need to go straight to the receiver. The TV's optical is for TV broadcast television.
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post #5 of 14
10/25/08 at 8:36am
Thread Starter 
I was really hoping to simplify things, passing a single cable from the room where the output equipment is located to the TV room.

I don't mean to be rude to somebody trying to help me, but it seems almost unimaginable for a TV to pass through AC-3 from ATSC HDTV but not DTS that's fed in digitally from HDMI.

Is there some more background as to why the TV wouldn't simply pass the digital data through?
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post #6 of 14
10/25/08 at 9:13am
ATSC mandates AC-3, and your TV has an AC-3 decoder in it. It therefore signals the transmitting HDMI device that is understands AC-3 (and PCM stereo), so these signals can be decoded as well as sent forward. The TV does not know what DTS is, so it cannot handshake the Apogee into allowing DTS to be sent, as that would result in no audio from the TV. Somehow your TV would need to operate as a "repeater" rather than a receiver for the HDMI signal, but it may not offer such an option, espcially since you want the TV to receive and display the video part of the HDMI payload.
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post #7 of 14
10/25/08 at 10:02am
Thread Starter 
I'm a little mystified on the DTS failure to pass-through. In the past, I have been able to pass DTS correctly through any pure digital system, even if they lack explicit support for DTS. The idea is that DTS can be handled like any LPCM data would be. For example:
http://www.macosxhints.com/article.p...05020515441690

On the premise that the TV can at least handle AC-3, I went ahead and fed some AC-3 data down the line. The receiver reports no data when I pass it though the TV. A direct connection to the receiver shows AC-3 data being processed properly.

This makes me think that the digital stream I'm passing is being corrupted en route (I can't see why, the TV is set to passthrough only), or (more likely) some bit on the SPDIF stream specifies the data type and thus the TV drops it from being passed through as "unknown."

It looks like I may be able to transcode into pretty much any format I want and dump it out the SPDIF line. It's just that I don't know that any format will be passed through properly.

I may investigate trying to "disguise" DTS audio as LPCM so that the TV doesn't block it, but then the receiver may not process disguised data properly.
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post #8 of 14
10/25/08 at 10:33am
Thread Starter 
OK, I've now tested playing a WAV file that contains DTS data. This means my audio output device is pumping out LPCM data that just happens to contain DTS.

A direct connection to the reciever results in perfect quality audio.

Routing the connection through the apogee and TV results in the obnoxious static-like noise that is encoded DTS.

The nearest I can tell, between the apogee and the TV, something is altering the audio in a way that prevents the DTS from being decoded by the receiver.

I would love any ideas...
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post #9 of 14
10/25/08 at 11:49am
HDMI devices need to handshake to communicate what singal can be send via HDMI. Your TV will tell the other end it can only accept 2-ch PCM or AC3, nothing else because that's all your TV can process.

Don't use your TV's SPDIF out as audio switch. It doesn't work for most TVs. They are only designed to output ATSC audio anyway.
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post #10 of 14
10/26/08 at 11:13am
Quote:
Originally Posted by blueandwhiteg3 View Post

OK, I've now tested playing a WAV file that contains DTS data. This means my audio output device is pumping out LPCM data that just happens to contain DTS.

A direct connection to the reciever results in perfect quality audio.

Routing the connection through the apogee and TV results in the obnoxious static-like noise that is encoded DTS.

The nearest I can tell, between the apogee and the TV, something is altering the audio in a way that prevents the DTS from being decoded by the receiver.

I would love any ideas...

While DD and DTS can travel the same S/PDIF ipe as PCM, there are indeed flags that identify the data as non-audio, so that a pure DAC will not try to present the raw coded data as audio. Sounds nasty. So yes you can fool the pipe as you did, since newer AV decoders have learned to decipher not only the flags but the actual data coded formats.
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post #11 of 14
10/26/08 at 12:11pm
Why do you need TV audio? Can't you just use your receiver for audio? That's what I do, and my life is simple.
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post #12 of 14
10/26/08 at 2:43pm
Thread Starter 
I want to use the receiver for all audio. But it only has one optical and one coaxial digital input. And it doesn't auto-switch.

The TV, on the other hand, always is viewing whatever I want to hear. Thus the optical pass-through on the TV seemed ideal to use.

The complaint here is that between the Apogee and TV, streams flagged as DTS/AC-3 get flat out dropped, and streams flagged as LPCM but containing DTS get corrupted such that the receiver cannot pick out a usable DTS stream. Directly feeding the same LPCM stream to the receiver works great.
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post #13 of 14
7/8/09 at 12:47pm
I've been struggling with what may be the same problem. A couple of things I've come across that make me wonder if the Panasonic TH-42PZ80U really can do the passthrough:

in that manual, on page 42, I quote
"If you connect equipment that has "HDAVI Control 2" (or more) function to the TV with HDMI cable, the sound from DIGITAL AUDIO OUT terminal can be output as multi channel surround."

and, from http://www.amazon.com/Panasonic-Viera-TH-42PZ80U-1080p-Plasma/dp/B00142MUDS/ref=sr_1_1?ie=UTF8&s=electronics&qid=1246951218&sr=8-1" , a reviewer (M. Hadeishi "synthetic zero") wrote
"Another thing I really like about the set is it passes through Dolby 5.1 sound from the HDMI connector to the optical digital out --- which I then pass into my Logitech Z-5500 THX speakers (an inexpensive way to get surprisingly good home theater sound) --- the Sharp didn't pass through the signal from our PS3 Blu-Ray player. "


My current theory is that the PS3 transmits this HDAVI data and then the TV allows the digital passthrough. HDAVI, however, is only supposed to be for remote-control functions, so this seems rather odd. HDAVI is Panasonic's name for CEC, see http://en.wikipedia.org/wiki/HDMI

Another curious note from page 38 of the manual:
"When audio from other equipment connected to this unit via HDMI is output using "DIGITAL AUDIO OUT" of this unit, the system switches to 2CH audio. (When connecting to an AV amp via HDMI, you can enjoy higher sound quality.)"

That contradicts page 42. I guess I'll have to call Panasonic's customer service http //www.panasonic.com/accessibility/aces_cus.asp and see if they can tell me anything...

manual: http://service.us.panasonic.com/OPERMANPDF/TH42PZ80U.PDF

FYI: the forum won't let me give URLs since this is my first post
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post #14 of 14
7/8/09 at 1:51pm
Thread Starter 
I concluded the apogee adaptor was not properly passing the data. I did a number of tests and found it has some kind of limitation... it seems it arbitrarily assumes the sample rate, or doesn't handle some sampling rates, or corrupted certain sample rates.

I tried it versus another audio extractor, and then shipped the box back with extreme disdain. I didn't test exhaustively against my TV again.

My final solution was very simple: run coaxial audio cable. It was actually a very healthy solution - the TV outputs optical to the receiver, the HTPC outputs coaxial digital, so I can switch nicely between them.

I'm very happy overall, although I still don't know whether the Panasonic TV passes through from HDMI properly. AC3 via optical is fine.

Beyond that... I don't have a ton more to contribute.
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