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Subject: AC-3 Encoded Stereo vs PCM Encoded Stereo
Posted by: rtbond
Date: 6/2/2005 7:34:33 AM

Folks,

Compression issues aside, when creating a DVD video disc, is AC-3 or PCM encoded audio perferred? I believe in using Vegas/DVD-A my stereo recording have always been AC-3 encoded.

With a PCM encoded DVD video disc I've discovered that I get no audio from my home theater connected (via the TOSLINK-S/PDIF) DVD video player. It appears the player disables the stereo (RCA) ouput if a TOSLINK cable is connected to the video player. Disconnect the TOSLINK cable from the video player and audio is heard.

So, is this an unusual DVD video player behavior? While this is a single data point, does this suggest AC-3 encoded stereo is a better way to go to minimize field DVD-video player/home theater playback issues?

Thanks

--Rob

p.s.. I did find by changing the DVD video player's Digital Audio menu setting from "ALL" to "PCM Only" then I can hear the PCM encoded stereo DVD audio track via the TOSLINK fed signal, unfortunately when I insert a Dobly Digital encoded commercial movie in the player my home theater receiver no longer indicates it is receivering Dobly Digita audio, but rather only PCM audio.

Subject: RE: AC-3 Encoded Stereo vs PCM Encoded Stereo
Reply by: alk3997
Date: 6/2/2005 1:33:00 PM

Your results don't seem typical to me. I think some of this may be misinterpretting what you wrote, so let me provide what I think should be happening...

The PCM encoded DVD-V audio should be sending a signal very similar to a CD output through the Toslink. This is a 2-channel, 16-bit signal. The difference is that a CD is always 44.1-kHz sampling whereas a DVD is likely 48-kHz sampling. There are also discs that output at 96-kHz. I believe the DVD-V spec requires support of 16 bit, 48-kHz PCM. Many early DVDs defaulted to the PCM audio track and AC3 had to be menu selected.

For AC-3 2-Channel (or 5.1), the bitrate is less than that needed for PCM. This is the advantage of using AC3. The reason for using PCM is audio fidelity. AC3 is roughly the equivalent of one CD channel of data. So, the only real trade-off is audio fidelity versus bitrate.

DVD-Audio discs have a completely separate set of audio requirements and I won't get into that here.

In your case, I suggest you look at what sampling rate and bit level you are sending as a PCM signal. It may be that your receiver can't handle that combination. Try encoding at 2-channel, 48-kHz, 16-bit.

If that doesn't work try a CD and see how that plays. It would be one more clue as to what is going on.

Andy

P.S. It is your receiver that auto-selects digital versus analog inputs. The DVD player has no clue whether the audio is hooked up to anything (the exception to this being an IEEE-1394 connection or a DENONLink connection).

Message last edited on 6/2/2005 1:36:48 PM by alk3997.
Subject: RE: AC-3 Encoded Stereo vs PCM Encoded Stereo
Reply by: rtbond
Date: 6/2/2005 6:33:36 PM

>P.S. It is your receiver that auto-selects digital versus analog inputs.
>The DVD player has no clue whether the audio is hooked up to
>anything (the exception to this being an IEEE-1394 connection or
>a DENONLink connection).

I do not believe this is the case. With a PCM stereo DVD in the DVD video player I get no sound. Disconnecting the S/PDIF digital audio connect from the back of the DVD player results in audio (sent via the analog stereo output of the DVD player). Repeat the experiment but disconnect the S/PDIF cable from the back of the A/V receiver (with the other end still connected to the DVD player) and still no audio. Hence my assertion that it is the DVD video player that is disabling the stereo analog output if a cable is plug into the S/PDIF port of the DVD player.

Subject: RE: AC-3 Encoded Stereo vs PCM Encoded Stereo
Reply by: alk3997
Date: 6/3/2005 8:46:00 AM

The only problem with your theory is that S/PDIF uses non-duplex (one way) transmissions. If you are using optical Toslink, you can check this by seeing if the red laser light is being transmitted.

Of course, don't look directly at the light but instead look at the reflection on another surface. As I'm reminded occasionally there is no such thing as safe laser light of any type.

The only thing I can think of is that there is some type of switch in the DVD player to tell it when the S/PDIF cable is inserted. What type of DVD player and receiver are these?

Also I should correct myself on one item from the earlier append. The AC3 bitrate is lower than a single CD channel. A single CD channel is about 705kbps. DVHS AC3 can be as high as 640kbps (typical is 576kbps). DVD is usually 448kbps. Broadcast is less. In all cases these are less a single PCM channel and less than DTS bitrates.

Andy

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