Dolby Surround Mode: which one for WAV->AC3 2.0?

[leonid_makarovsky]

I am recording a VHS to DVD. VHS is a rock concert. On its cover it says HI FI Stereo. However, when I listened to it in Movie Dolby Digital mode, I noticed the audience in surround speakers. In Vegas, when I convert WAV to AC3 2.0, there're 3 Dolby Surround Modes: Not Indicated, No Dolby Surround Encoded, Dolby Surround Encoded. I tried all 3. When I play all 3 in Stereo mode, I don't seem to hear any diference between them. When I play all 3 in Dolby Digital Movie or Dolby Digital Music Mode, I also don't seem to hear the difference betwen any of 3. The only thing is when I start playing, only DVD with Dolby Surround Encoded mode by default goes to Dolby Digital Movie mode in my receiver. The rest 2 (No Dolby Surround Encoded and Not Indicated) go to Stereo mode. So what is the real diference between all 3 of them and especially between Not Indicated and No Dolby Surround Encoded? If I choose Dolby Surround Encoded and play it Stereo mode, does it give me true Stereo (as if I played the original VHS) or does it downmix Dolby Surround in Stereo mode during playback? Also what if I choose Not Indicated and then it turns out that the concert was Dolby Surround encoded, will playback give me true Dolby Surround if I choose Dolby Digital Music mode or Dolby ProLogic? And yes, I also tried to encode it in 5.1 surround mode, but didn't like the final result 'cause surround left and surround right channels just repeated front left and front right channels respectively. Thanks.

--Leonid

[tebasuna51]

I am recording a VHS to DVD. VHS is a rock concert. On its cover it says HI FI Stereo.
Then your source is just stereo. To encode like Dolby Surround in the cover must say "Dolby Surround".

However, when I listened to it in Movie Dolby Digital mode, I noticed the audience in surround speakers.
When you force the receiver to do the Dolby Surround decode some random output can be listened. If your receiver have DSP (Digital Signal Processing) you must use this functions instead Dolby decoder.

I also don't seem to hear the difference betwen any of 3.
Exact, there are not difference betwen any of 3.

The only thing is when I start playing, only DVD with Dolby Surround Encoded mode by default goes to Dolby Digital Movie mode in my receiver. The rest 2 (No Dolby Surround Encoded and Not Indicated) go to Stereo mode. So what is the real diference between all 3 of them and especially between Not Indicated and No Dolby Surround Encoded?
The audio are exact, only a flag in header say to your receiver switch automatically to Dolby Digital Movie mode for the encoded like Dolby Surround, the rest 2 do nothing.


If I choose Dolby Surround Encoded and play it Stereo mode, does it give me true Stereo (as if I played the original VHS) or does it downmix Dolby Surround in Stereo mode during playback?
There are nothing to downmix, you have only 2 channels and is played like the original VHS.

Also what if I choose Not Indicated and then it turns out that the concert was Dolby Surround encoded, will playback give me true Dolby Surround if I choose Dolby Digital Music mode or Dolby ProLogic?
Never, your source is plain stereo.

And yes, I also tried to encode it in 5.1 surround mode, but didn't like the final result 'cause surround left and surround right channels just repeated front left and front right channels respectively.
If you don't have DSP functions you can read
Stereo-to-Surround Conversion Guides (http://forum.doom9.org/showthread.php?t=83752)

[leonid_makarovsky]

Thanks a lot for your explanation. By the way, there's no such thing as Movie Dolby Digital on my receiver. I read it wrong. The name of the mode is Dolby Prologic IIx Movie. I hope that doesn't make a difference.

Then one question... what is the difference between NO DOLBY SURROUND ENCODED and NOT INDICATED? Thanks.

--Leonid

[tebasuna51]

By the way, there's no such thing as Movie Dolby Digital on my receiver. I read it wrong. The name of the mode is Dolby Prologic IIx Movie. I hope that doesn't make a difference.
I suppose something like this. My comments are valid for Dolby Surround/ProLogic I/ProLogic II/ProLogic IIx decoders.

Then one question... what is the difference between NO DOLBY SURROUND ENCODED and NOT INDICATED?
There are not useful difference. From Dolby documentation:
Not Dolby Surround The bitstream contains information that was not encoded in Dolby Surround.
Dolby Surround The bitstream contains information that was encoded in Dolby Surround. After Dolby Digital decoding, the bitstream is decoded using Pro Logic.
Not Indicated There is no indication either way.

[shadowhaze]

When you force the receiver to do the Dolby Surround decode some random output can be listened.

I've been using my receiver Prologic II setting to simulate 5.1 surround for 2.0 ac3 audio, but given your comment above, I'd like to know it I'd get better quality converting ac3 2.0 to 5.1 instead. It's a little confusing since my understanding is that ac3 encoding=loss of quality - whether it's ac3 -> wav/lossless -> ac3 or simply ac3 -> ac3.


If you don't have DSP functions you can read
Stereo-to-Surround Conversion Guides (http://forum.doom9.org/showthread.php?t=83752)

My receiver's DSP settings ("arena", "Jazz club", "concert hall", etc.) can not be accessed simultaneously w/ ProLogic II. I've read some threads on adding DSP (usually reverb) when converting 2.0 to 5.1, but I can't tell if it's necessary for proper conversion, or just personal preference.

Thanks.

[tebasuna51]

I've been using my receiver Prologic II setting to simulate 5.1 surround for 2.0 ac3 audio, but given your comment above, I'd like to know it I'd get better quality converting ac3 2.0 to 5.1 instead.
The question is:
Your ac3 2.0 is Dolby ProLogic encoded?

- If yes, you can obtain the better quality using the Prologic II decoder at play time.
- If not, you can test the Prologic II decoder (based in math relations between the 2 physical channels), the DSP functions (based in delays, echoes, etc.) or make a conversion 2.0 -> 5.1 with your personal tastes.
It's a little confusing since my understanding is that ac3 encoding=loss of quality - whether it's ac3 -> wav/lossless -> ac3 or simply ac3 -> ac3.
To transcode ac3 -> ac3 always need decode the ac3 to uncompressed format and after encode to ac3 (or other format).
- The firts pass, ac3 decode, is a lossless process and can be stored in a wav file or send directly to the ac3 encoder (without intermediate wav files, but with the same effect).
- The second pass may be any signal processing like the upmix 2.0 -> 5.1 ( or downmix, resample, filtering, timestretch, etc...).
Here you can lose or gain quality (filtering noise for instance) but is your personal choice or a system requirement (resample 44100 to 48000 for DVD audio track, ...)
- The last pass is recompress to ac3 (or other lossy codec) and here you always lose quality. This pass is unnecessary when your receiver make the process at play time.

My receiver's DSP settings ("arena", "Jazz club", "concert hall", etc.) can not be accessed simultaneously w/ ProLogic II.
DSP's and ProLogic are different methods to obtain the surround signals like I say before.

I've read some threads on adding DSP (usually reverb) when converting 2.0 to 5.1, but I can't tell if it's necessary for proper conversion, or just personal preference.
Always personal preferences and source dependent.

My last recommendation:
If you have a receiver with ProLogic and DSP's to convert to surround at play time the upmix 2.0 -> 5.1 is unnecessary for you.
Of course you can test your personal choices, make a backup of original audio and experiment ...

Really the last recommendation:
In upmix make 2.0 -> 5.0 (and use a LFE empty for compatibility if you need) because your receiver make the bass redirection to the subwoofer (or like you have configured) from all the channels, not only from LFE, and is useless make the extraction by software previously (only can be a source of troubles).

[shadowhaze]

:thanks: for the detailed reply. It was very helpful.

The question is:
Your ac3 2.0 is Dolby ProLogic encoded?
- If not, you can test the Prologic II decoder (based in math relations between the 2 physical channels), the DSP functions (based in delays, echoes, etc.) or make a conversion 2.0 -> 5.1 with your personal tastes.

None of the ac3 sources are Prologic encoded - I should have mentioned that. Some are Dolby Digital and some are just 2.0 stereo (b/4 ac3 conversion).

To transcode ac3 -> ac3 always need decode the ac3 to uncompressed format and after encode to ac3 (or other format)...
The last pass is recompress to ac3 (or other lossy codec) and here you always lose quality. This pass is unnecessary when your receiver make the process at play time.

I'm not sure what you mean by "this pass is unnecessary" since the tracks will muxed w/ video and transferred to dvd. I thought you couldn't use a straight wav file for MPEG2/DVD.

DSP's and ProLogic are different methods to obtain the surround signals like I say before.

(*smacks forehead*) For some reason, I never thought about DSPs that way. Normally, I avoid using any DSPs on pre-recorded material.

My last recommendation:
If you have a receiver with ProLogic and DSP's to convert to surround at play time the upmix 2.0 -> 5.1 is unnecessary for you.
Of course you can test your personal choices, make a backup of original audio and experiment ...

Really the last recommendation:
In upmix make 2.0 -> 5.0 (and use a LFE empty for compatibility if you need) because your receiver make the bass redirection to the subwoofer (or like you have configured) from all the channels, not only from LFE, and is useless make the extraction by software previously (only can be a source of troubles).

Please clarify. I'm not sure if you're suggesting it's a waste of time to convert 2.0 to 5.0/5.1 since my receiver can do it ("Last recommendation"), or if I should go ahead and convert 2.0 to 5.0 ("Really the last recommendation"). BTW - I don't know if this makes any difference with your advice, but my receiver's pretty good, though a few years old (THX Ultra cert. THX sur.EX, DTS ES/Neo, etc.).

Thanks again for your help.

One last thing, I saw one of your posts with a multichannel db analysis. What did you use? Is there any free software or VST plugin (for Cubase) you recommend for analyzing track(s) db levels?

[tebasuna51]

None of the ac3 sources are Prologic encoded - I should have mentioned that. Some are Dolby Digital and some are just 2.0 stereo (b/4 ac3 conversion).

Ok. But also stereo mp3 can be ProLogic encoded if is a backup from a 5.1 using free tools like BeSweet-Azid (Gordian) or AviSynth-BeHappy (MEGUI).

I'm not sure what you mean by "this pass is unnecessary" since the tracks will muxed w/ video and transferred to dvd. I thought you couldn't use a straight wav file for MPEG2/DVD.

If you preserve the original audio and lets your receiver make the upmix at play time, you don't need recode to ac3 (lossy pass)

Please clarify. I'm not sure if you're suggesting it's a waste of time to convert 2.0 to 5.0/5.1 since my receiver can do it ("Last recommendation"), or if I should go ahead and convert 2.0 to 5.0 ("Really the last recommendation").

The best recommendation: experiment yourself.

One last thing, I saw one of your posts with a multichannel db analysis. What did you use? Is there any free software or VST plugin (for Cubase) you recommend for analyzing track(s) db levels?
Is the free SoundOut (http://forum.doom9.org/showthread.php?t=120025) plugin for AviSynth.