[geezer]

I recently read a review of another brand of DAC. It was a 16 bit device which, when fed by a 24 bit transport, exhibited noise when played at low volume.

I recently ordered the AVA Ultra DAC, which is a 16 bit machine, and I'm now wondering whether I need to worry about the source that will be feeding it; i.e., must it too be 16 bit?

Geezer

[miklorsmith]

TacT said their 2.2XP which is a 24-bit output was compatible with 16-bit machines.  They tested my unit and also an external, 16-bit DAC on my unit.  Both tests reported no issues with my TacT.  However, the Altmann and also an Ack! 2.0 exhibited the same problem in my system.

TacT and Altmann said there should be no compatibility issues.  Before I had the XP upgrade to my 2.2X, I was using a Monica 2 DAC and I'm pretty sure I had another Ack! 2.0 with no problems.  I know the XP upgrade has many significant modifications but whether this was the culprit I don't know.

TacT also said they've had no reports of any compatibility problems with the XP and many of their users have external DACs.  Of the cross section of all the DACs in existence, relatively few are 24 bit.  So, lots of folks are using their TacTs with 16 bit DACs without problems.

My month-long odyssey of trying to get to the bottom of this problem resulted in no conclusive story.  Generally, I think the 24 bit/16 bit issue isn't a big deal but you might get unlucky and lose the unlottery.

[Charles Calkins]

Geezer:
 Don't bother with a transport. Get yourself a slimdevices SB3 setup. IMHO that is the best and maybe the only way to listen to CD's. SB3 digital out to the ultra dac and you got yourself something that will knock the socks off of any transport.

                               Cheers
                              Charlie

[modular747]

I recently read a review of another brand of DAC. It was a 16 bit device which, when fed by a 24 bit transport, exhibited noise when played at low volume.

I recently ordered the AVA Ultra DAC, which is a 16 bit machine, and I'm now wondering whether I need to worry about the source that will be feeding it; i.e., must it too be 16 bit?

Geezer

There is no such thing as a "24 bit transport".  Transports are just what they say - they spin the disk and read and send the digital format on the disc to the DAC which converts the digital to analogue.  If you are playing a CD, it is the "source" and by definition 16 bits.  Anything that "up converts" the 16 bits to pseudo 24 bits (you can't create more info than is encoded in the disc) is more than a "transport" and a ridiculous waste of money.  Another marketing scam for the technically ignorant.

[modular747]

Hey, I'm technically ignorant, and want to be clear on what you're saying. Could someone explain exactly what this means in terms of players that claim, specifically, HDCD playback ability? If there is no such thing as a player that plays HDCD differently, then one is left to conclude that outfits like, say, Reference Recordings are doing a deliberate "scam" on consumers, since no one could actually benefit from their recording method, which sounds kinda wacky. Or am I unknowingly changing the subject here.

You aren't distinguishing between a player and a transport.  The player has it's own DAC.
HDCD (do they even make them anymore?) is a 24 bit integrated player system for playing 24 bit HDCD discs, not 16 bit CD's. The "digital out" on these players is (by anti-copy law) down-converted to 16 bits, so that if you try to use an external DAC, you get "only" 16 bit resolution (regardless of the capabilities of the external DAC). 

[miklorsmith]

My TacT preamp upconverts to 24 bits to preserve resolution using its internal volume control.

Thanks for the kind words.

[geezer]

Thanks to all. Several points have been clarified for me.

Cheers,
Geezer

[modular747]

You seem to imply (your wording) that HDCD players don't play 16-bit CDs, but of course they do...

I didn't mean to imply that they can't play standard CDs, but that they don't get any more "information" or resolution out of them than a standard CD player (or transport feeding an external DAC). Earlier posts mentioned potential problems using "24 bit transports" playing standard CD's with a 16 bit external DAC.  The point is that if the transport if playing a CD or HDCD, the digital output has to be 16 bits either because the source is 16 bits or because of the required down-conversion.

[modular747]

My TacT preamp upconverts to 24 bits to preserve resolution using its internal volume control.

You can't up-convert a 16 bit data stream to true 24 bits - you can't generate real information not in the original.  The up-converter creates a pseudo 24 bit digital format by interpolating the extra bits, providing no real additional resolution.  The "digital volume control" can only subtract musical information from the real 16 bits.

[miklorsmith]

Take it up with TacT.  This is the reason they give for the process.  Forgive me if I put more credence in their knowledge than yours.

[modular747]

Take it up with TacT.  This is the reason they give for the process.  Forgive me if I put more credence in their knowledge than yours.

You "give credence" to what TacT says as being objective science rather than marketing and advertising copy because...(nothing to with with the fact that you've already bought into their hype, of course).  This is the problem of knowing nothing about the basic engineering and science behind a technical issue, accepting statements about it on authority and then parroting them as if you understood them.  This isn't religion or magic; you decide what's true with knowledge and reason.  Try doing some reading about digital technology - there's plenty out there that doesn't require an engineering or math degree.

[miklorsmith]

Oooo, attack the poster, I love that game.

What TacT's reasons are for converting their signal to 24 bits are irrelevant to the post at hand, which was wondering if there might be a compatibility issue between a 16 bit device and one that outputs a 24-bit signal.  I have first hand experience with this issue and was commenting to the best of my knowledge.

That is what the post was about and should be about.  Casting aspersions of others' gullibility and stupidity as an aside to an honest question is a crappy way to make friends and influence people.

[modular747]

which was wondering if there might be a compatibility issue between a 16 bit device and one that outputs a 24-bit signal.  I have first hand experience with this issue and was commenting to the best of my knowledge.

If you read my previous posts, there aren't any devices which "outputs a 24 bit signal" to an external DAC.  24 bit HDCD player using 24 bit HDCD or DVD audio discs are required to down-convert the digital output to 16 bits, 44.8 khz sampling rate.  Any transport or player using standard CD's are limited to 16 bits because that's all there is...  The only exceptions are certain Pioneer DVD-HDCD  players which can feed a true 24 bit digital output via an HDMI connection to their own brand amps which contain DAC chips (they don't make free standing DACs). This signal is encrypted to prevent copying and would be useable only by other HDMI signal processors.  There is no need to worry about feeding a Van Alstine DAC a 24 bit signal from any player/transport as this can't happen.  Any device that claims to "up-convert" a 16 bit CD to 24 bits can't increase the theoretical limits of the dynamic range, S/N ratio and frequency response beyond what's on the 16 bit disc without artificially expanding and interpolating (guessing) the "missing" bits.  True 24 bit systems have theoretically (but not necessarily audibly) better specs because they use the original 24 bit digital recording tapes without having to down-convert them to 16 bits for the CD format.  Once they're down-converted, the additional information is lost forever - it can't be "recreated" by up-conversion, only simulated.

[miklorsmith]

OK.  Maybe the TacT preamps are the only source of (albeit artificially enhanced) 24 bit digital audio in existence.  In that case, don't sweat it unless you're using a TacT preamp.

[modular747]

I'm not clear where you have the TacT preamp in the signal chain.  A preamp normally gets the analogue output signal from the DAC - it doesn't send any signal back to it.

[avahifi]

Hey Modular, keep up the good work, you are explaining it better than I can.

Now tackle the issue that 1-bit CD players can only read bits at their maximum internal clock speed and with a typical 10 meg clock speed, they can only read 500 of the 65,000 bits on the CD per cycle at 20,000 Hz on a one pass basis.  Their sample and hold circuits give a "fake" perfect 16-bit readout on repetitive test signals, but not so nice on real music. Or why we use a 16-bit parallel processing system.

Oh well.

Frank Van Alstine

[miklorsmith]

The TacT is a digital preamp which can accept analog or digital signals.  With analog, the signal is converted to digital for internal processing to manage subwoofer/main crossovers, room correction, and parametric EQ.  With a digital input, the incoming signal is converted to 24/96 for all processing.  The unit has inboard DACs or can send digital output for outboard conversion.

As far as not being able to "create" resolution from thin air, is this different from upsampling/oversampling with frequency?  Obviously it isn't the *same* thing, but I definitely hear differences in resolution between upsampling and nonoversampling DACs.

[modular747]

The TacT is a digital preamp which can accept analog or digital signals.  With analog, the signal is converted to digital for internal processing to manage subwoofer/main crossovers, room correction, and parametric EQ.  With a digital input, the incoming signal is converted to 24/96 for all processing.  The unit has inboard DACs or can send digital output for outboard conversion.

As far as not being able to "create" resolution from thin air, is this different from upsampling/oversampling with frequency?  Obviously it isn't the *same* thing, but I definitely hear differences in resolution between upsampling and nonoversampling DACs.

In this case you would be using the preamp to receive the analogue output of the (Van Alstine) DAC.  The word length (bits) and sampling rate of the original source and the DAC are immaterial and there is no compatibility issues with the digital circuits in the preamp.  The preamp then reconverts the analogue back to digital for its signal processing.  This is true 24/96 (if that's the format used) as it isn't up-converting a digital signal but A-D converting from analogue.   However, each time a signal is converted back and forth form analogue->digital->analogue->digital->analogue, etc., there always some loss of accuracy as no converters are even theoretically perfect.  A case can be made for doing D-A conversion as far downstream as possible and doing signal processing in the digital domain, but doing an extra conversion just for a digital volume control strikes me as a solution without a problem.

[miklorsmith]

No, digital source to digital TacT, then digital out to DAC.  No ADC.

[modular747]

No, digital source to digital TacT, then digital out to DAC.  No ADC.

So, are you using an external DAC (specifically Van Alstine) between the preamp and power amp?  If this works at all, the digital output of the preamp has to be 16 bits (44 - 48 kHz) otherwise it won't convert.  If you're using the internal DAC or another brand of external DAC, I'm not sure how that relates to this discussion and the Van Alstine forum.

Also, digital volume controls work by "bit stripping and shifting" - progressively lopping off the least significant bit.  In a 16 bit system, a 36 db attenuation reduces the signal to 10 bits.  Thus, the more the volume is turned down, the higher the noise, distortion, and compression of dynamic range - pretty much the opposite of what an analogue volume control does.   Up-converting the signal to 24 bits (and using a 24 bit DAC) leaves more bits to play with and may reduce this effect (on paper anyway), but the overall effect is the same.