On 11/27/05, Steve Adeff <[EMAIL PROTECTED]> wrote: > On Sunday 27 November 2005 16:26, Michael T. Dean wrote: > > Steve Adeff wrote: > > What exactly does this mean? If you're doing AC-3 passthrough from > > Myth, won't the S/PDIF output be as bit-perfect as was sent to you by > > the digital TV broadcaster/DVD production house? Or, are you saying > > that some sound card's mangle the stream they're attempting to pass > > through? > > > > Mike > > Yes, 'mangle', because the chipset will do freq/bitrate conversion if needed > to match the abilities of the output circuitry. If the output circuits can > handle 24/196 then no change is needed and the chip just sends out the > signal. Creative's output circuits can't handle 24/196 so the signal gets > reprocessed to match its output (20/92 iirc). This is whether the signal is > DD/DTS or PCM.
Here's the thing. The S/P-DIF specifications state that signals have to be transferred at either 44.1Khz/16-Bit or 48Khz/16-Bit for PCM, DD/DTS (Which is a raw mix from the source), or AC-3 (5.1 Channel, 48/44.1Khz, 16-Bit). However, Creative's SB Live! cards add to this, by being able to decode (On-card) DD5.1 to AC3 (And multiple outputs). But this is optional, and can be switched off (At least in the Windows Drivers, I don't know about the Linux ones). If it's switched off, and the S/P-DIF is addressed directly, then the format that's presented (DTS/DD/AC-3) will be piped directly to the S/P-DIF and out to the Amp via the S/P-DIF RCA or TOSLink connector. If the input stream is from a DVB/ATSC source, it should be in AC-3 (Or DD, I'm not certain), And if it's from a DVD source, it'll be in 48Khz PCM/DD/DTS, which means that there will be no resampling. If the source is an PVR card, make sure it's set to capture at 48Khz, and there will be no resampling (As it'll get decoded into a PCM stream, that'll then hit the soundcard and S/P-DIF untouched). If the source is an Analog card (BTTV), then you may be stuck with 32Khz, and will have to upsample to 44.1/48Khz. This may introduce upsampling problems, BUT you will have this no matter what card you use, as the S/P-DIF forces to be upsampled. As another point, 99% of cards have an internal "Working Frequency", which means there can be resampling artefacts on virtually every soundcard out there. AC'97 (The general soundcard spec) states soundcards must accept and output 48Khz, and most cards use that. SB Audigy's use 92Khz (That's 48Khz x 2) so they can very simply convert into and back from S/P-DIF's 48Khz recommended frequency. However, as this is all real Audiophile details, the chances of any of this coming out in subjective listening (Without $100,000 sound equpiment) is very very slim indeed. -- Robert "Anaerin" Johnston _______________________________________________ mythtv-users mailing list mythtv-users@mythtv.org http://mythtv.org/cgi-bin/mailman/listinfo/mythtv-users
