Bit perfect music playback with GBPVR

[mwj]

Hi!

Is it confirmed that the ALC883 chip in Windows XP plays back bitperfect with digital out? I have that chip in my Asus P5K-VM and would like to not go the route with ASIO or KS if not needed. I do find the audio somewhat lacking when compared to that of S/PDIF from my CD player so I suspect something is happening...

I am in the process of turning my entire CD collection to FLAC and put the CD player and CD records away. But the sound must be equally good from the HTPC! Bit perfect is the key.

I have the ALC883 and indeed it is capable of bit perfect audio for both video (AC3, DTS) and for CD (44.1KHz). The video part was just a matter of getting the drivers configured correctly, but the only way I could get bit perfect CD audio was by using foobar2000 with kernel streaming (which may have required a plugin).

If you find another way to get bit perfect CD audio, let me know.

[Barsk]

I have the ALC883 and indeed it is capable of bit perfect audio for both video (AC3, DTS) and for CD (44.1KHz). The video part was just a matter of getting the drivers configured correctly, but the only way I could get bit perfect CD audio was by using foobar2000 with kernel streaming (which may have required a plugin).

If you find another way to get bit perfect CD audio, let me know.

Well, I *may* be there. But I am not sure. My audio driver has a tray icon which may open a Sound Manager. In there is a Audio I/O tab (if my memory serves me). There you hav an option to select 44.1 KHz. With this setting I am getting 44.1 kHZ frequency to the receiver. So no transcoding to 48 kHz at least. When playing DTS or AC3, the audio is 48 kHZ although the Audio I/O tab i still set to 44.1, and this is perfect.
I guess this is possible due to the fact that alc883 has a hardware mixer and can thereby bypass the Windows kmixer.

However when I listen to a flac track on the computer with S/PDIF to the receiver I get a very slightly less perfect sound as when listening to digital audio out from the standalone CD player. I had to listen very carefully for a long time to come to this conclusion. I had actually hoped they would be 100% equal.

So, there is probably still something happening the digital audio stream. That needs to be investigated further. I will check with foobar and see how it sounds in there. The problems *may* be caused by a lower quality S/PDIF interface on the computer compared to that on my expensive CD player, i.e jitter. Or maybe the cabling plays in. The S/PDIF cable from the computer is of ok quality but not excellent - as the other.

All in all, the change in setting from 48 kHz to 44.1 kHz made wonders. Now I just need that last little bit of quality and I am home.

[mwj]

Well, I *may* be there. But I am not sure. My audio driver has a tray icon which may open a Sound Manager. In there is a Audio I/O tab (if my memory serves me). There you hav an option to select 44.1 KHz. With this setting I am getting 44.1 kHZ frequency to the receiver. So no transcoding to 48 kHz at least. When playing DTS or AC3, the audio is 48 kHZ although the Audio I/O tab i still set to 44.1, and this is perfect.
I guess this is possible due to the fact that alc883 has a hardware mixer and can thereby bypass the Windows kmixer.

However when I listen to a flac track on the computer with S/PDIF to the receiver I get a very slightly less perfect sound as when listening to digital audio out from the standalone CD player. I had to listen very carefully for a long time to come to this conclusion. I had actually hoped they would be 100% equal.

Yes, I was in that state at one point, too. What you need to do is find a test track that will only play if you are bit perfect at 44khz. If you can get a DTS CD (not a DVD, but a real DTS CD) you can test with that; unfortunately, there weren't many of those made so you may not have one. The other thing you can do is search for a test track called something like SURROUND_TEST.avi or something similar. It is a DTS track that will only play if you are bit perfect end to end, otherwise you will just get noise. Search the forums for threads on bit perfect playback. If you can't find it, PM me with your email address and I will email it to you.

[Barsk]

Yes, I was in that state at one point, too. What you need to do is find a test track that will only play if you are bit perfect at 44khz. If you can get a DTS CD (not a DVD, but a real DTS CD) you can test with that; unfortunately, there weren't many of those made so you may not have one. The other thing you can do is search for a test track called something like SURROUND_TEST.avi or something similar. It is a DTS track that will only play if you are bit perfect end to end, otherwise you will just get noise. Search the forums for threads on bit perfect playback. If you can't find it, PM me with your email address and I will email it to you.

I will lock for a DTS test track, I have seen these forum posts somewhere. That where my next thing to try actually. If I cannot find it I'll PM you.

On another note, DTS and AC3 playback you claim to have gotten bit perfect? I have only concluded that I am at 48 kHz, not what other stupid stuff is made to the audio. In a forum post at Doom9 the author of eac3to describes that most decoders do some changes to the stream, like volume or dynamics ,which defeats my purpose of sending the data unchanged, bit perfect to my receiver. So if you have any hints to what decoders to use and how to set them up I am most interested.

Regards
Barsk

[mwj]

On another note, DTS and AC3 playback you claim to have gotten bit perfect? I have only concluded that I am at 48 kHz, not what other stupid stuff is made to the audio. In a forum post at Doom9 the author of eac3to describes that most decoders do some changes to the stream, like volume or dynamics ,which defeats my purpose of sending the data unchanged, bit perfect to my receiver. So if you have any hints to what decoders to use and how to set them up I am most interested.

It's my understanding that AC3 and DTS streams are not decoded if you are sending them to a receiver over S/PDIF. I believe that if you are using a S/PDIF cable and your receiver lights up the "Dolby Digital" or "DTS" indicator, you are bit perfect for AC3 and DTS.

[Barsk]

It's my understanding that AC3 and DTS streams are not decoded if you are sending them to a receiver over S/PDIF. I believe that if you are using a S/PDIF cable and your receiver lights up the "Dolby Digital" or "DTS" indicator, you are bit perfect for AC3 and DTS.

Hmm, I have that. But you have to have some sort of decoder for the AC3 and DTS streams, and they may be set up for S/PDIF output. However there may still be a lot of tweakings going on with the data and you will still end up with the indicators lit on the receiver. Try AC3 Filter. There are a lot of changes that you can make regarding dynamic compression, normalisation etc, even when using digital out.

The fact that I was using digital out was the reason I was severely mislead with the CD audio stuff and S/PDIF. I believed I was bit perfect, but I was not.
I fear the same goes for DTS and AC3.

EDIT: Found a bunch of DTS tracks. http://www.sr.se/multikanal/english/e_index.stm

[Barsk]

My recent findings.

With foobar and the kernel streaming plugin I am indeed playing bit perfect! Checked with the DTS wav files and they play fine in foobar. Also my flac tracks and CD Audio is played back perfectly now. I cannot sense a difference in sound quality compared to digital out from my CD player to the receiver versus digital out from the HTPC. So that was the goal achieved. Sound is really good now and finally I can retire my old CD player for sure!

However, I want this for all audio out! I watch a lot of TV on the HTPC and most of that is MPEG audio or similar 2 channel audio. That too needs to be put out to the digital out in bit perfect ways. How to achieve that?

[Barsk]

More findings. Actually the DirectShow filter ReClock can use kernel streaming mode, so I downloaded that, installed, configured and checked with the DTS wav file. It worked! I can now play bit perfect audio from any application, even from GB-PVR.
In order to do this, enable Kernel Streaming for PCM audio (NOT for DTS/AC3) in Reclock and also disable media playback modification (play at original speed). Also set the Audio renderer in GB-PVR to Reclock and you are there!

One problem persists. The ALC883 (Realtek) drivers lets me specify the outpu sample frequency, so I set that to 44.1 kHz. And that works perfectly for CD audio, flac, mp3 etc. And when playing DTS or AC3, the realtek driver lets that through automatically in 48 kHz. Perfect too. However MPEG audio from my TV recordings are 48 kHz natively. They will NOT get passed unless i set the output to 48 kHz. So Realtek autosense DTS/AC3 and lets it through unaltered. PCM audio however uses the Realtek config setting and resamples to that setting.

For now I have the setting at 44.1 kHz which makes music, AC3/DTS play perfect. The TV recordings are getting resampled however. The problem to solve is how to get Realtek ALC 883 to pass 44,1 and 48 kHz PCM audio unaltered without fiddling with the Audio I/O settings.