How does digital attenuation in sound cards work?

[MuFu]

Take, for example, the M-Audio Revo. This card reportedly has a "twenty-channel mixer with 36-bit internal resolution" and 24-bit DACs. I assume this means bitstreams are resolution-enhanced (and filtered?) before DSP occurs. Say you reduce the volume by 36dB (*1/64). Every time you scale by a factor of 1/2 you effectively lose a bit, right? So it's a 30-bit signal that's then fed to the DACs, which I presume crop the redundant LSBs.

Are claims that digital attenuation reduces the effective source resolution of a recording unfounded when dealing with hardware such as the M-Audio card? If the process is as described above, it seems unlikely that even 24-bit recordings could be noticeably affected, let alone most CDs. I've been told many times on AV forums that you should never feed a power amp directly from a soundcard because of (amongst other things) degraded resolution and SNR at attenuated levels.

MuFu.

 

[MuFu]

Anybody? Russ?

I did a search on AVS and there's barely any mention of digital attenuation. It is pretty common there to see HTPC users running analogue feeds directly to their power amps with no analogue preamp. I hope this means they realise there's no problem in doing so (and their ears agree!).

C'mon eggheads, get your egg-on.

MuFu.

 

[thegrommit]

Try the forums at 3Dsoundsurge:

http://www.3dss-forums.com/cgi-bin/wwwthreads/wwwthreads.pl?Cat=

 

[Simon F]

Anybody? Russ?

I did a search on AVS and there's barely any mention of digital attenuation. It is pretty common there to see HTPC users running analogue feeds directly to their power amps with no analogue preamp. I hope this means they realise there's no problem in doing so (and their ears agree!).

C'mon eggheads, get your egg-on.

MuFu.

Mufu,
I'd ask one of my colleagues for your who is into audio recording (both analogue and digital) but he's just gone off on hols... sorry.

 

[MuFu]

Thanks guys.

I've been in a few professional recording envioronments where monitors are fed directly from digital desks with no analogue preamps (apart from the fixed-gain stages used for level-matching etc). I supect this may be another area - like poncy cables, component isolation platforms, bi-wiring etc - where pro audio guys frown a little at the crazy world of high-end audiophilia.

MuFu.

 

[RussSchultz]

More likely you should never feed a power amp from a soundcard because soundcards are all crappy in terms of electrical isolation, and most of them have DC offsets and make a step function when turning on (makes very loud pop on amplified systems). I wouldn't be suprised if some of them don't conform to the proper line levels, also.

Generally, I imagine the M-Audio doesn't scale your samples up (well, maybe to 30 bits), but uses that overhead for clip avoidance. (20 channels added together can overflow 5-6 bits if they're all at full scale when summed).

But, the overall question, does digital attenuation hurt SNR? The answer would be yes, by definition. If we remove analog gain from the question, your system's noise floor will be constant. Your signal's peak will be less with digital attenuation, so the SNR must be lower.

 

[MuFu]

More likely you should never feed a power amp from a soundcard because soundcards are all crappy in terms of electrical isolation, and most of them have DC offsets and make a step function when turning on (makes very loud pop on amplified systems).

Yeah that's the main problem, IMHO. Some cards aren't too bad in this respect - Audigy 2 for example. The Revo and most Soundstorm-based mobos are pretty horrifying though. It makes power amp auto-on/off very tricky to set up for HTPCs.

I wouldn't be suprised if some of them don't conform to the proper line levels, also.

Creative products have been known to be very dodgy as far as the co-ax digital feed goes (causes all sorts of problems with source switching in receivers and there was some talk of them actually causing damage through use of non-compliant voltages). Perhaps one day they'll just buy-out the IEC. Not sure about the analogue outs but I assume most of the pro/prosumer cards are ok in this respect.

Generally, I imagine the M-Audio doesn't scale your samples up (well, maybe to 30 bits), but uses that overhead for clip avoidance. (20 channels added together can overflow 5-6 bits if they're all at full scale when summed).

That makes sense.

But, the overall question, does digital attenuation hurt SNR? The answer would be yes, by definition. If we remove analog gain from the question, your system's noise floor will be constant. Your signal's peak will be less with digital attenuation, so the SNR must be lower.

That too.

Cheers Russ,

MuFu.

 

[Tim Murray]

Just because somebody mentioned an M-Audio card... Newegg is selling the M-Audio Delta 410 for $116 rather than the MSRP of... what... $300? $350? Either way, good deal. Just bought two myself