What is being decoded where?

2010-01-11 18:51:13

I have a new audio setup that involves a PC running iTunes into an Onkyo TX-SR707 receiver. The receiver has Burr Brown codecs and sounds great.

The PC needs to connect to the receiver via analog and digital. This is because the receiver cannot play digital sources in Zone 2, only Zone 1.

I am using Virtual Audio Cable to allow the PC to output both analog and digital streams at the same time (I think).

So here is the signal path:

iTunes > VAC Cable 1.

VAC Cable 1 splits into Realtek HDMI out driver, and Realtek generic audio driver.

The HDMI driver is going out via HDMI to the Onkyo, where I assume the Burr Brown codec is converting it to analog?

The Generic Audio driver I assume is using some king of motherboard soundcard-based codec to transform to analog, and sending the signal via RCA cables to the Onkyo. The Onkyo amplifies the analog signal but does apply any codec?

Edit: Most of the iTunes content is lossless, but some is MP3, and I don't know how this affects what is being decoded where...

Do I have this understood correctly? thanks.

What is being decoded where?

Reply #1 – 2010-01-12 00:00:49

I don't know anyting about your Onkyo, and I don't use VAC, but I'll try to give you some info...

Depending on what format you are playing and depending on the set-up & connections, the decoding and digital-to-analog conversion can be done either the computer or the receiver.... Or, just the decoding can be done by the computer, and the digital-to-analog conversion be done by the receiver.

Of course with the analog connection, the decoding (decompression) and digital-to-analog conversion is done by your soundcard/driver.  Your reciever can optionally apply DSP effects to the analog signal and it can decode "analog" Dolby Surround from a 2-channel analog signal.    For true (discrete) 5.1 surround, you must use 6 separate analog inputs (or a digital connection)

HDMI is digital.  The digital data over the HDMI connectinon can be either compressed or uncompressed.  When you play a DVD on yorr computer, typically Dolby AC3 and DTS (compressed) digital streams are simply "passed through"  the soundcard, out over the digital connection (S/PDIF or HDMI), and decoded in the receiver.  And of course, the receiver's DAC (digital-to-analog converter) is used.  (Most home-theater receivers can't decode MP3 or other 'audio-only' formats.)

For uncompressed audio, (typically 2-chanel stereo) the digital signal may be re-sampled & mixed so that you can get all of the mixed audio that normally comes out of your computer speakers.

When you play an MP3, FLAC, ALAC, or other "non-movie" format in iTunes, the file will be decompressed and converted to an uncompressed PCM stream by a CODEC in the computer.  The PCM stream will be sent to your soundcard's DAC (to the analog outputs) and to the digital outputs.   

What is being decoded where?

Reply #2 – 2010-01-12 13:15:53

Thank you, that's quite helpful.

I did not understand the distinction between a deompression to a PCM stream, vs. a separate operation to convert digital - analog.

The deeper you dig the more complicated it gets... especially when you consider how much just "works" automatically.

What is being decoded where?

Reply #3 – 2010-01-12 16:42:30

For your specific question, the answer is simple. iTunes is decoding everything, and that's that.

Linear PCM is the uncompressed format in which digital audio in the PC (and almost everywhere else) is encoded. In order to save space, you can also encode in other formats (like MP3, or Apple Lossless). Since the PC can't process these formats directly, they have to be converted (commonly referred to as "decoded") into LPCM, then the DAC can convert to analog and output to the amp/speakers.

You can decode to LPCM either in the PC or in the receiver, but there is only a limited set of formats you can transport to your receiver (a.k.a. "bitstream") to be decoded there. Typical are Dolby Digital and DTS formats, which can be transferred via SPDIF. HDMI audio allows for other formats as well, such as Dolby TrueHD and DTS-HD which are the audio formats of bluray discs. WMA Pro and AAC can also be bitstreamed but I've never seen those applied. Another one is MLP which was the DVD-Audio format (TrueHD is based on it). Your receiver may or may not decode all or some of these.

What motherboard and audio device (sound card) are you using by the way?

What is being decoded where?

Reply #4 – 2010-01-12 21:31:40

Andy, it looks like I've found the right guy again! ASUS M3A78-EM mobo with HD3200 chipset.

So iTunes is decoding everything from whatever audio file format it is stored in, to LPCF via a codec. And right before the PCF data hits the motherboard soundcard analog RCA ports it is run thru a mobo based DAC for Analog conversion?

... but then when that linear PCF data is sent to the Onkyo via HDMI is the Onkyo running it thru its Burr Brown DACs to convert to analog? So it is not converted to Analog at the motherboard?

Thanks.

What is being decoded where?

Reply #5 – 2010-01-12 21:49:25

Yeah, all that is right, except it's PCM (pulse-code modulation) not PCF. And I wouldn't worry a lot about what kind of Brown the DAC's are

What is being decoded where?

Reply #6 – 2010-01-12 21:52:57

oh I didn't notice you were the same guy in the other thread.