DAC IV stages Options

[Yahzi]

How important are DAC IV stages in the design of a DAC? I've heard they can cause audible differences. Is this true? I'm not a DAC designer, but I assume there are those who understand the technical merits. Is it a big deal?

[John_Siau]

Take a look at published specifications for D/A converters, DVD players, sound cards, iPod, etc. Many only have an 80 to 90 dB SNR. This means that the noise will be audible in an A/B/X test if the peak music levels exceed 80 to 90 dB SPL (easily achieved).

Worse yet, most D/A converters are preceded by a digital volume control. If 20 dB of digital attenuation is dialed in (very typical), the SNR will degrade by 20 dB. In many systems, the user will need to apply 20 dB of digital attenuation to achieve a playback level that peaks at 100 dB SPL. Under these conditions the converter noise may reach 30 to 40 dB SPL. It is not hard to hear the noise floor drop from 30 of 40 dB SPL to less than 0 dB SPL in an A/B/X test.

I have an ABX tester, and it is very easy to pick out D/A converters that have high noise floors.

If one wishes to use a digital volume control, the D/A converter will need an extra 20 dB SNR to keep noise inaudible at loud playback levels. If we want to achieve "CD" quality and use 20 dB of digital attenuation, then 116 dB SNR is required. Under these conditions, noise may still become audible if playback peaks exceed 96 dB SPL.

D/A converters with >116 dB SNR are few and far between. Noise alone is a dead giveaway when comparing D/A converters.

This post has been edited by John_Siau: Feb 20 2013, 22:57

[saratoga]

Take a look at published specifications for D/A converters, DVD players, sound cards, iPod, etc. Many only have an 80 to 90 dB SNR.

You're exaggerating. Most quality players (iPods included) are very nearly quantization noise limited over line out. 80dB is quite rare and would be exceptionally bad for a name brand device. The Apple stuff will give you 15.5 effecitve bits over headphone out with full analog volume control for not all that much money.

D/A converters with >116 dB SNR are few and far between. Noise alone is a dead giveaway when comparing D/A converters.

Yeah but in reality it doesn't make much difference since you never use that dynamic range. CDs using > 80dB dynamic range are far rarer then D/A converters with < 80dB dynamic range.

[John_Siau]

80dB is quite rare and would be exceptionally bad for a name brand device. The Apple stuff will give you 15.5 effecitve bits over headphone out with full analog volume control for not all that much money.

15.5 effective bits when the volume control is set at maximum, and to the best of my knowledge, none of the Apple devices have analog volume control (please correct me if I am wrong). With 20 dB digital attenuation, 15.5 effective bits becomes 12.2 effective bits. The problem is that significant amounts of digital attenuation are often used at normal listening levels, and the converter noise is often audible.

Yeah but in reality it doesn't make much difference since you never use that dynamic range. CDs using > 80dB dynamic range are far rarer then D/A converters with < 80dB dynamic range.

CD and high-resolution low-noise recordings do exist, and if these are used in an ABX test, the noise floors of many converters should be audible (on the basis of calculated SPL of the noise).

[saratoga]

80dB is quite rare and would be exceptionally bad for a name brand device. The Apple stuff will give you 15.5 effecitve bits over headphone out with full analog volume control for not all that much money.

15.5 effective bits when the volume control is set at maximum, and to the best of my knowledge, none of the Apple devices have analog volume control (please correct me if I am wrong).

All portable electronics have analog volume control, Apple's included. You couldn't buy an mp3 player that had digital volume control if you wanted to. They aren't made.

With 20 dB digital attenuation, 15.5 effective bits becomes 12.2 effective bits. The problem is that significant amounts of digital attenuation are often used at normal listening levels, and the converter noise is often audible.

Yeah but no one does this so it doesn't matter.

Edit: Although I don't mean to imply that analog volume control imposes no SNR hit. I'm sure at the lowest volume the SNR on an ipod is much worse. But at reasonable volumes its more then sufficient, which is what I was trying to say.

Yeah but in reality it doesn't make much difference since you never use that dynamic range. CDs using > 80dB dynamic range are far rarer then D/A converters with < 80dB dynamic range.

CD and high-resolution low-noise recordings do exist, and if these are used in an ABX test, the noise floors of many converters should be audible (on the basis of calculated SPL of the noise).

Yes, but people don't buy CE to do ABX tests of specially selected samples. They buy them to listen to music. With enough effort I can find samples that break MP3, AAC, Vorbis, etc. Doesn't mean I dislike those formats. It just means I can break them if I try.

This post has been edited by saratoga: Feb 21 2013, 20:11

[John_Siau]

All portable electronics have analog volume control, Apple's included. You couldn't buy an mp3 player that had digital volume control if you wanted to. They aren't made.

Absolutely incorrect!

Almost all of these products all have buttons or cap-touch buttons (or sliders) that control volume up and down. In almost all cases, the volume control is a digital multiplier. Very few consumer products still have analog volume controls (variable resistors) in the audio path. DSP is cheap and is already required to do MP3 decoding.

[saratoga]

All portable electronics have analog volume control, Apple's included. You couldn't buy an mp3 player that had digital volume control if you wanted to. They aren't made.

Absolutely incorrect!

Name one such product. Just one.

Almost all of these products all have buttons or cap-touch buttons (or sliders) that control volume up and down. In almost all cases, the volume control is a digital multiplier.

You've obviously never reverse engineered any of these devices if you think this.

Edit: I mean think about what you're claiming. Any modern device can set the line out and headphone out volume independently. Do you really think theres two stereo DACs included just so that they can use independent digital volume adjustments? No, that'd be nuts, the cost would be huge. They have independent volume control because the headphone amp and line out amp volume is software controlled.

This post has been edited by saratoga: Feb 21 2013, 23:36

[John_Siau]

All portable electronics have analog volume control, Apple's included. You couldn't buy an mp3 player that had digital volume control if you wanted to. They aren't made.

Absolutely incorrect!

Name one such product. Just one.

Almost all of these products all have buttons or cap-touch buttons (or sliders) that control volume up and down. In almost all cases, the volume control is a digital multiplier.

You've obviously never reverse engineered any of these devices if you think this.

Edit: I mean think about what you're claiming. Any modern device can set the line out and headphone out volume independently. Do you really think theres two stereo DACs included just so that they can use independent digital volume adjustments? No, that'd be nuts, the cost would be huge. They have independent volume control because the headphone amp and line out amp volume is software controlled.

Wrong, but your conclusion that two DACs would be required is correct!

The Apple devices use Cirrus CLI158881 Stereo Codecs which are similar to the CS42L73. These devices have two stereo DACs, one stereo ADC, and a digital mixing engine. Such chips form the audio core of almost all portable devices. Digital gain control is ubiquitous.

See:

http://www.cirrus.com/en/products/cs42l73.html

This post has been edited by John_Siau: Feb 22 2013, 15:16

[Arnold B. Kruege...]

All portable electronics have analog volume control, Apple's included. You couldn't buy an mp3 player that had digital volume control if you wanted to. They aren't made.

Absolutely incorrect!

Name one such product. Just one.

The Apple devices use Cirrus CLI158881 Stereo Codecs which are similar to the CS42L73. These devices have two stereo DACs, one stereo ADC, and a digital mixing engine. Such chips form the audio core of almost all portable devices. Digital gain control is ubiquitous.

See:

http://www.cirrus.com/en/products/cs42l73.html

Agreed. The usual chips we find in mainstream portable digital audio player and smart phones and tablets generally has at last 4 DACs and 2 ADCs. Even though they are not all used by the specific products, this is also true of the SOC that is the heart of the economically-priced Sansa Clip+ and Fuze.

[saratoga]

From NWAVGuy's dissection and review of the Clip:

http://nwavguy.blogspot.co.uk/2011/02/sans...p-measured.html

DAC linearity is important because most portable devices have digital volume controls that reduce the signal before the DAC

ha, hes wrong. I somehow missed that reading the review.

And I'm new here and have never taken a DAP apart, but requiring that someone you disagree with undertake the entire burden of proof while you should be assumed correct by default seems a bit much.

How exactly do I prove that something doesn't exist? I can point out that I've worked on drivers for lots of these devices and not one of them uses digital gain control, but I can't really prove it since I haven't worked on every single device in existence, just a lot of them.

Agreed. The usual chips we find in mainstream portable digital audio player and smart phones and tablets generally has at last 4 DACs and 2 ADCs. Even though they are not all used by the specific products, this is also true of the SOC that is the heart of the economically-priced Sansa Clip+ and Fuze.

No, those are 1 stereo DAC devices. Two is actually rare in low power devices, and off hand I can't think of any dedicated MP3 players that do that. I think that Cirrus part above is aimed more at tablets and higher power devices, so probably they can afford a little more logic.

[scuttle]

From NWAVGuy's dissection and review of the Clip:

http://nwavguy.blogspot.co.uk/2011/02/sans...p-measured.html

DAC linearity is important because most portable devices have digital volume controls that reduce the signal before the DAC

ha, hes wrong. I somehow missed that reading the review.

No. You have failed to understand your own post. You wrote:

You couldn't buy an mp3 player that had digital volume control if you wanted to. They aren't made.

That the chip used in the Clip (assuming this is your evidence for your contention - you didn't bother saying why anybody should listen to you) has analog AND digital volume control still makes NWAVguy right and you wrong. Read what you wrote!

And if that chip isn't your evidence, then what is? Are people supposed to say "Yes; this man is angry and can't use apostrophes - clearly he must be in the right!"? No.

And I'm new here and have never taken a DAP apart, but requiring that someone you disagree with undertake the entire burden of proof while you should be assumed correct by default seems a bit much.

How exactly do I prove that something doesn't exist? I can point out that I've worked on drivers for lots of these devices and not one of them uses digital gain control, but I can't really prove it since I haven't worked on every single device in existence, just a lot of them.

Yes: your claim was inherently unprovable - which makes it silly - but if you could name a couple of major players then that would at least be indicative.

And you can claim to have written drivers, but so what? The Internet is full of people who claim to be special forces, CIA agents, and aliens - especially when they think it will help them win some silly debate. NWAVGuy is a reasonable expert witness because everyone here knows that he is competent enough to design the ODAC; all we know about you is that you can type.

[saratoga]

No. You have failed to understand your own post. You wrote:

You couldn't buy an mp3 player that had digital volume control if you wanted to. They aren't made.

The Clip does not have digital volume control, so even if you take that statement hyper-literally, its still correct (although its also not what I meant).

That the chip used in the Clip (assuming this is your evidence for your contention - you didn't bother saying why anybody should listen to you) has analog AND digital volume control still makes NWAVguy right and you wrong. Read what you wrote!

And if that chip isn't your evidence, then what is? Are people supposed to say "Yes; this man is angry and can't use apostrophes - clearly he must be in the right!"? No.

Well I wrote the volume control code used in that review, and its open source, so you can read it if you want:

http://git.rockbox.org/?p=rockbox.git;a=co...81ad204720e0eb0

Yes: your claim was inherently unprovable - which makes it silly - but if you could name a couple of major players then that would at least be indicative.

I don't follow you. Why is pointing out that Apple, Sandisk and virtually all other companies use analog volume control unprovable or silly?

And you can claim to have written drivers, but so what? The Internet is full of people who claim to be special forces, CIA agents, and aliens - especially when they think it will help them win some silly debate. NWAVGuy is a reasonable expert witness because everyone here knows that he is competent enough to design the ODAC; all we know about you is that you can type.

You can easily google my username and verify that I've written this stuff. Or just search on these forums. I post about this stuff a lot, and plenty of people here know me, NWAVguy included.

[John_Siau]

Well I wrote the volume control code used in that review, and its open source, so you can read it if you want:

http://git.rockbox.org/?p=rockbox.git;a=co...81ad204720e0eb0

Rockbox uses the MAS3539F MP3 decoder and Codec.

This chip has only one D/A and one A/D as Saratoga has been claiming.

One block diagram (page 7 of the data sheet) shows a volume control feature in the headphone amplifier (following the D/A converter). This implies that the unit has some analog volume control capability.

However, a more detailed block diagram on page 8 shows digital mixing and audio processing before the D/A converter (in the digital domain).

The truth is that the MAS3539F uses both analog and digital volume control. The following is a quote from page 10 of the data sheet:

"To minimize quantization noise, the main volume control
is automatically split into a digital and an analog
part. The volume range is −114...+12 dB with an additional
mute position. A balance function is provided."

Register 00 11 controls the main volume. This is "split
between a digital and an analog function" (see page 51 of the data sheet).

Bottom line, some MP3 players may use a combination of analog and digital gain control (see MAS3539F) . Others only use digital gain control (see CS42L73).

[saratoga]

Bottom line, some MP3 players may use a combination of analog and digital gain control (see MAS3539F) .

Like I said above, modern devices do have digital mixers. So what? However, the MAS isn't a modern device. Its a 1990s hardware MP3 decoder with no CPU. It dates back to a time when mobile CPUs were too slow to process audio. If you're really going to try and argue this, do you want to pick something from at least this millenium

Others only use digital gain control (see CS42L73).

Did you see above where I showed that this is not true?

Like I said before, you're just wrong about "almost all" of these devices using software volume control. I've never seen a portable device that uses software volume control, and apparently you have not either. Or if you have, feel free to tell me about it . . .

[John_Siau]

Like I said before, you're just wrong about "almost all" of these devices using software volume control. I've never seen a portable device that uses software volume control, and apparently you have not either. Or if you have, feel free to tell me about it . . .

A digital volume control can be implemented in hardware or software. It is often implemented in hardware and the software merely sends register commands to the hardware (as in the Rockbox). Rockbox code exists for MAS3539F hardware, and this is why I used this chip as an example. I assumed your code was written to support this chip. Did you write code for a different chip? If so, which one(s). What chip is used in the clip?

[saratoga]

Like I said before, you're just wrong about "almost all" of these devices using software volume control. I've never seen a portable device that uses software volume control, and apparently you have not either. Or if you have, feel free to tell me about it . . .

A digital volume control can be implemented in hardware or software. It is often implemented in hardware and the software merely sends register commands to the hardware (as in the Rockbox). Rockbox code exists for MAS3539F hardware, and this is why I used this chip as an example. I assumed your code was written to support this chip. Did you write code for a different chip? If so, which one(s). What chip is used in the clip?

Rockbox is an operating system that runs on many dozens of different devices each with different DACs. The MAS is the first device it ever ran on, being the hardware decoder of one of the very first MP3 players ever made. But you misunderstand the meaning of those registers. The MAS does not provide software access to it's PCM data. There are hardware registers for scaling data because its impossible to do it any other way, and scaling data is often needed (for example, volume normalization). Volume control is supposed to be implemented using a variable gain amplifier (and in fact we do implement it this way).

There are many different clip players. The first used the AS3525 which had a DAC similar to the AS3514 integrated. The newer ones (that are being discussed here) use an unknown chip dubbed (by us) the AS3525v2, which uses a DAC that is related to the AS3543 (although the register maps are not quite identical).

[John_Siau]

Rockbox is an operating system that runs on many dozens of different devices each with different DACs. The MAS is the first device it ever ran on, being the hardware decoder of one of the very first MP3 players ever made. But you misunderstand the meaning of those registers. The MAS does not provide software access to it's PCM data. There are hardware registers for scaling data because its impossible to do it any other way, and scaling data is often needed (for example, volume normalization). Volume control is supposed to be implemented using a variable gain amplifier (and in fact we do implement it this way).

There are many different clip players. The first used the AS3525 which had a DAC similar to the AS3514 integrated. The newer ones (that are being discussed here) use an unknown chip dubbed (by us) the AS3525v2, which uses a DAC that is related to the AS3543 (although the register maps are not quite identical).

I fully understand the purpose of the registers in the MAS, and I understand that the MAS does not give the CPU access to the PCM data. But, digital "scaling" is digital volume control. It may not be the primary volume control system, but it still performs part of the volume control function. 6 dB of gain reduction via "scaling" will move the audio 6 dB closer to the noise floor. This digital scaling can also cause truncation if it is not dithered.

A variable gain amplifier may also attenuate the audio without having much impact on the output noise.

In either case, if the signal is decreased without also decreasing the output noise, then the effective SNR is reduced.

An analog pot (not found on many modern devices) has the ability to reduce the signal and the noise simultaneously (thus preserving the SNR of the audio). Digital volume controls and variable gain amplifiers do not replicate the function of a true analog gain control.

[saratoga]

Rockbox is an operating system that runs on many dozens of different devices each with different DACs. The MAS is the first device it ever ran on, being the hardware decoder of one of the very first MP3 players ever made. But you misunderstand the meaning of those registers. The MAS does not provide software access to it's PCM data. There are hardware registers for scaling data because its impossible to do it any other way, and scaling data is often needed (for example, volume normalization). Volume control is supposed to be implemented using a variable gain amplifier (and in fact we do implement it this way).

There are many different clip players. The first used the AS3525 which had a DAC similar to the AS3514 integrated. The newer ones (that are being discussed here) use an unknown chip dubbed (by us) the AS3525v2, which uses a DAC that is related to the AS3543 (although the register maps are not quite identical).

I fully understand the purpose of the registers in the MAS, and I understand that the MAS does not give the CPU access to the PCM data. But, digital "scaling" is digital volume control. It may not be the primary volume control system, but it still performs part of the volume control function. 6 dB of gain reduction via "scaling" will move the audio 6 dB closer to the noise floor. This digital scaling can also cause truncation if it is not dithered.

You're mixing up truncation and scaling. Truncation (with or without dithering) happens no matter what. All of these devices operate at 32 bit internally (except truly ancient devices like the MAS). You always truncate your 32 bit PCM coming out of your MP3/FLAC/whatever decoder down to 16 bit (or 24 bit in the case of PCs). Note that this happens no matter how volume control is implemented. So its no surprise you see the signs of that under a scope. A quality 16 bit device should basically be expected to do that since dithering is uncommon.

Now you can also scale the waveform. In rockbox we do this too, although only by small amounts (a few dB for EQ precut, replaygain, etc). This doesn't change the situation with truncation, since that still has to happen. Finally, at the end you have volume control, which is done by changing gain to preserve SNR.

A variable gain amplifier may also attenuate the audio without having much impact on the output noise.

In either case, if the signal is decreased without also decreasing the output noise, then the effective SNR is reduced.

An analog pot (not found on many modern devices) has the ability to reduce the signal and the noise simultaneously (thus preserving the SNR of the audio). Digital volume controls and variable gain amplifiers do not replicate the function of a true analog gain control.

Yes of course. Like I said before:

However, while they'll all give you nearly 16 bit limited performance into a line out, they tend to have fairly limited amplifiers (ignoring Apple, Sandisk which are very good). They also have essentially fixed noise floors that are independent of volume. So a more useful approach is to think about them in terms of the impedance and sensitivity that will give good performance. The noise floor puts a limit on sensitivity, since very high sensitivity headphones will produce more acoustic noise, while the finite output impedance limits how low of an impedance can be driven without distortion.

The fixed noise floor kills you. So while analog gain is better then digital, it doesn't save you running into the noise floor at low volumes. So SNR does decrease, although slower then 6dB/bit of digital. The flip side of this though is that going to more effective bits doesn't really help you (unless you do it by reducing the device's analog noise floor). My point, therefore, was that since we operate under these constraints at all but the very highend, worrying about DAC design is not very productive. You need better analog amplifiers after the DAC or its all for nothing.