DAC modding

[ajithlal]

What is modding?
Is it possible to increase audio quality by replacing 24 bit DAC by 32 bit in CD/DVD player?

It happened to find a site which deals with modded oppo players..such as
Oppo DV-980H, DV-981HD and DV-983 mods with optional 32 bit DAC and 32 bit/211Khz Upsampler
Could any one put more light on this?
Ajith

 

[venkatcr]

Modding refers to home grown electronics enthusiasts who take a stock product such as a CDP, iPOD, amplifier etc, and make 'modifications' internally. These modifications are supposed to improve the performance of the unit. Modifications start from just changing the fuse, to replacing the wiring, and many times creating a shorter electrical path inside, or even changing the whole circuitry. Most common are replacing stock parts with what the modifier thinks are better such as transistors, capacitors, DSP chips, etc. All these modifications will void the warranty on the product. For all repairs you have to go back to the person who had modified the unit, as the original manufacturer would not know what to do.

Is 32 bit DAC better?

A digital signal is a series of finite numbers. A analogue signal is continuously varying quantity. The DAC take the finite numbers and converts them to continuous signals. In audio, this is usually a low electrical voltage that can be amplified and played on speakers.

In practice, the sequence of numbers is sent to the DAC with a clock signal. When the DAC receives a number, it generates a output voltage that it maintains till it receives the next set of numbers. When the next numbers comes in, the DAC rapidly changes the voltage to match the value represented by the new numbers. When this done what you will get is a set of harmonics represented by as a staircase shaped output. This creates multiple harmonics above the required frequency as represented by the Shannon-Nyquist theorem. To smoothen this, the DAC uses a low pass filter that acts as a reconstruction processor. Unfortunately, the filter may have a mild roll-off effect at the highest frequencies. Since it is very difficult to recreate the original signal precisely, all DACs end up with some amount of approximation. Mind you, these are very very small differences we are talking about, and mostly in the higher frequencies.

In an constant endeavour to improve upon their performance, DAC manufacturers use higher sampling frequencies and higher resolutions.

The resolution (16, 24, 32 bits) are the number of possible levels that the DAC is designed to reproduce.

The sampling frequency measures the speed at which the DAC can produce the output. As per the Shannon-Nyquist theorem, a DAC MUST, at the least, sample at twice the frequency of the original signal. If you take audible signals as being a max of 20KHz, a DAC must operate at a minimum of 40KHz. A Redbook Audio CD sample data at 44.1 KHz. So a DAC must work at a minimum of 88.2 KHz.

Given these basic premises, a higher resolution and sampling frequency will produce better results. That is at least the understanding in the Industry.

Cheers

 

[ajithlal]

Thank u so much for detailed explanation.
Ajith

 

[gopi]

venkat, finally i give up. I also have to join the huge number of people being amazed by your knowledge and the patience to write such detailed replies You are indeed a great add to this forum. I have question here. Some people consider that upsampling an input can be negative as it is seems to be adding data which wasn't there and hence can also have negative impact. So, in short purists seem to shy away from upsampling. Is this the case? Also, whats the difference between upsampling and oversampling?

 

[pnredkar]

Hi,

Venkat, despite your explanation, I am still skeptical.

DAC - Digital to Analogue converter takes a specific bit-width of input and converts it to an equivalent analogue signal. A 32-bit can take in 2^32 distinct discrete values and convert them to 2^32 different voltage levels.

Also if I am not mistaken, the sampling rate is important for an ADC. For a DAC the input it already in the digital domain. The numbers that are important here are
(1) Word length (24-bit or 32-bit) that represents the possible discrete values.
(2) Words per second that represents how closely the points on the waveform will be placed. In case of Redbook CD, this will be 44.1K words per second (Edit: I misread you. You say this is 88.2K words per second and you must be right. I have no technical knowledge about redbook-CDs)

Definitely, a 32-bit DAC will be better than a 24-bit DAC, provided that your can provide a 32-bit input. Now to get a 32-bit input, you have to change the entire digital chain to output 32-bit precision numbers. I don't think that modders do this.

Now, the simple way to get a 32-bit input is left zero padding. However, in this case you are not adding any information and do not get any gain from a higher precision DAC. [--- There is other one other thing that needs to be considered - Suppose that the digital pattern with all zeros corresponds to voltage 0.0 and that the digital pattern with all ones correspond to voltage 1.0V. In this case of zero-padded 24-bit word, the maximum voltage you are going to generate is (1/256)V. You may be playing with the dynamic range of your output! ---] (Edit: This last bit is not completely correct - See post below)

Now, this is entirely my thought process. I may be completely missing some trick in the book. But, I would be skeptical to get any modded player. If there were some improvements to be made, the original designer would have made it. It is about optimizing the whole chain. Optimizing one part of the chain (like the output stage) will have incremental benefits at best. Never a day and night difference.

Regards,
Prasad Redkar.

 

[pnredkar]

Now, the simple way to get a 32-bit input is left zero padding. However, in this case you are not adding any information and do not get any gain from a higher precision DAC. There is other one other thing that needs to be considered - Suppose that the digital pattern with all zeros corresponds to voltage 0.0 and that the digital pattern with all ones correspond to voltage 1.0V. In this case of zero-padded 24-bit word, the maximum voltage you are going to generate is (1/256)V. You may be playing with the dynamic range of your output!

The above is a problem if done the wrong way. And what I have explained is the wrong way

<Warning: Technical stuff>
Let me give an example of a 2-bit DAC converted to 4-bit DAC. The possible inputs with 2-bit DAC are "00", "01", "10" and "11" (binary equivalent of 0, 1, 2, 3). This would be converted to "0V", "0.25V", "0.5V" and "0.75V".

If we left pad them, the numbers are "0000", "0001", "0010" and "0011". With a 4-bit DAC the voltages become "0V", "0.0625V", "0.125V" and "0.1875V". This is the wrong way to do it.

However if we right pad, the numbers are "0000", "0100", "1000" and "1100" (equivalent decimal - 0, 4, 8, 12). A 4-bit DAC will give "0V", "0.25V", "0.5V" and "0.75V" as in the original case. This is the right way. But no advantage gained using 4-bit DAC.
</Technical stuff>

So then what do upsampling players such as CA 840C and Wadia players do? They upgrade the entire digital chain to produce higher precision digital samples at a higher rate. The computations are done at a higher precision. Also value are filled in between the time intervals using some intelligent algorithms. CA 840C input to the DAC is 32-bit precision at 384KHz while the Wadia does it at 24-bit precision at 1.4112MHz.

So, back to the original topic. I am a non-believer in modding until some the modder can scientifically explain me how he intends to improve the performance of a stock unit.

Case rested

Regards,
Prasad Redkar.

 

[odyssey]

Prasad
Actually, I believe the answer is not so simple. Definitely if the information is there then its best to pass it all, so in terms of bits, if the pre-recorded information has only 16 bits, what would padding extra bits do? Oversampling is a different story and there are two camps - Zanden, DCS, audio note some which believe that oversampling is better and some believe that NOS (non-OS) is best. I personally like them both for different kinds of music (I have a CDP where I can select this). In terms of modding, perhaps you referred to the bit resolution only, but if you take other mods that can be made - output caps, op-amps, quality of components such as resistors, etc there can be a huge difference in the sound quality of the DAC so much so that it can be mistaken for a completely different DAC at a different price. Even tube rolling for example can be thought of as such a mod for an output stage...
Recently I got an NOS DAC built to my specification with no op-amps and filters using black gate caps ($200 just for the caps), the sound is phenomenal and am sure it can go head to head with CDPs costing Rs 1 L.

cheers

 

[reignofchaos]

In an constant endeavour to improve upon their performance, DAC manufacturers use higher sampling frequencies and higher resolutions.

The resolution (16, 24, 32 bits) are the number of possible levels that the DAC is designed to reproduce.

The sampling frequency measures the speed at which the DAC can produce the output. As per the Shannon-Nyquist theorem, a DAC MUST, at the least, sample at twice the frequency of the original signal. If you take audible signals as being a max of 20KHz, a DAC must operate at a minimum of 40KHz. A Redbook Audio CD sample data at 44.1 KHz. So a DAC must work at a minimum of 88.2 KHz.

Given these basic premises, a higher resolution and sampling frequency will produce better results. That is at least the understanding in the Industry.

Cheers

The DAC doesn't need to work at double the frequency of the input data. It has to work at double the frequency of the original signal. So the DAC can work at 44.1kHz itself for redbook audio. It doesn't need to work at 88.2kHz. Upsampling does help certain types of DACs though. Also let me mention there are no 32 bit converters out there. The best available ones (non proprietory) are 24bit/192kHz.

As Sridhar mentioned, there are two camps in the industry - one that builds DACs using oversampling converters such as Ti (Burr Brown), Cirrus Logic (Crystal) and Analog Devices. The other camp is the non oversampling camp which is typically built with Philips TDA1543 or similar converters.

Having heard the sound of both these types of DACs, I can honestly say that the oversampling converters seem to have more detail resolution. However they are more prone to edginess and digital processing artifacts which might make them sound unnatural at times. Again it also depends on hundreds of other factors such as DAC topology and the likes. NOS DACs typically have a more analogue'ish sound closer to what one gets from Vinyl. They are typically not the detail champions but again there are a few exceptions. However they typically produce more realistic sounding voices.

What to buy depends on the rest of one's chain. There are good and bad examples of both oversampling and NOS dacs. For oversampling DACs, upgrading the clock to something higher precision and higher frequency makes a dramatic difference. Upgrading my DAC's clock from a 22.x MHz to a super high precision 33.x MHz TCXO did wonders to it.

 

[pnredkar]

Hi Sridhar and ROC,

I am not commenting on the merits and demerits of upsampling/oversampling etc.

The point is that there is an stock CDP which has a x-bit y-rate DAC. Does it make sense to replace the DAC with a >x-bit or >y-rate capable DAC. The entire digital chain of the player has been designed to supply x-bits at y-bit-rate. Hence upgrading just the DAC will not make a difference.

Having said that some modders use upsampling DACs for smoother outputs. Here I have a technical reason to believe it may make a difference. How much is another story. Similarly, better output caps, opamps, etc. are still believable claims. But, very few of the modification claims are backed by hard data or technical reasoning.

And I agree it is not simple at all!

Regards,
Prasad.

 

[reignofchaos]

Hi Sridhar and ROC,

I am not commenting on the merits and demerits of upsampling/oversampling etc.

The point is that there is an stock CDP which has a x-bit y-rate DAC. Does it make sense to replace the DAC with a >x-bit or >y-rate capable DAC. The entire digital chain of the player has been designed to supply x-bits at y-bit-rate. Hence upgrading just the DAC will not make a difference.

Having said that some modders use upsampling DACs for smoother outputs. Here I have a technical reason to believe it may make a difference. How much is another story. Similarly, better output caps, opamps, etc. are still believable claims. But, very few of the modification claims are backed by hard data or technical reasoning.

And I agree it is not simple at all!

Regards,
Prasad.

Agreed. There's no point only changing the DAC in a CD player. The entire circuitry right after the transport needs to be redesigned. One might get some mileage by replacing the passive components, the clock and the output stage opamps (if applicable).

 

[ajinkya]

Having read the thread, I'm curious what the Cambridge Audio DACMagic does then and why people would be buying it at all:
Cambridge Audio DacMagic Upsampling Digital to Analogue Converter Released

 

[pnredkar]

Hi Ajinkya,

The reason why you would use an external DAC:
(1) For the same input specs, the quality of the output (and hence the DAC) differs. External DACs typically have the better quality DAC chipsets.

(2) There is science behind upsampling. The intermediate (time-wise) values are interpolated intelligently, so that the control over the output waveform is greater. How much you actually like the sound of upsampling is entirely up to your personal choice.

(3) No moving parts, better isolation of components, etc.

The ways in which you would use the external DAC:
(a) Digital music stored on portable players, USB drives or PCs would benefit largely from such external DACs.

(b) CD transports (the cheaper versions, atleast) may have errors while reproducing the bits due the spin of the CD. If you can make a bit perfect copy of the CD using some software like EAC on your PC/USB drive, you may eliminate some of these transport errors. You now need only a DAC to complete your high-quality "CD player".

These are some of the points off the top of my head. There may be others.

Regards,
Prasad Redkar.

 

[ajinkya]

Prasad,

Thanks very much for the information. Yes, what you say makes sense. However, the upsampling part is still 'black magic' to me because all we are doing is substituting some curve (spline or other curve "du jour") in between the sampling instants. So information is being 'created', not recreated. Which makes me wary of the final sound. But as you rightly mention, that is upto individual taste. The same idea is used in HDTV upconversion as well and my eyes don't seem to be unhappy watching those, so why should my ears reject audio upsampling?
As an aside, anyone have an idea on what the price in rupees of the CA DacMagic would be?

 

[pnredkar]

The same idea is used in HDTV upconversion as well and my eyes don't seem to be unhappy watching those, so why should my ears reject audio upsampling?

Good comparison!

As an aside, anyone have an idea on what the price in rupees of the CA DacMagic would be?

The price in UK is 200GBP, in US it is $399. Looking at the prices of other CA products, I would expect it to be around Rs. 18000/-.

Regards,
Prasad Redkar.

 

[neo]

Prasad
Actually, I believe the answer is not so simple. Definitely if the information is there then its best to pass it all, so in terms of bits, if the pre-recorded information has only 16 bits, what would padding extra bits do? Oversampling is a different story and there are two camps - Zanden, DCS, audio note some which believe that oversampling is better and some believe that NOS (non-OS) is best. I personally like them both for different kinds of music (I have a CDP where I can select this). In terms of modding, perhaps you referred to the bit resolution only, but if you take other mods that can be made - output caps, op-amps, quality of components such as resistors, etc there can be a huge difference in the sound quality of the DAC so much so that it can be mistaken for a completely different DAC at a different price. Even tube rolling for example can be thought of as such a mod for an output stage...
Recently I got an NOS DAC built to my specification with no op-amps and filters using black gate caps ($200 just for the caps), the sound is phenomenal and am sure it can go head to head with CDPs costing Rs 1 L.

cheers

Hey! can pl share more info your DAC. Details?Spec?Availiblity?Cost?etc

 

[odyssey]

Its for personal use (at least so far) quite a few people have asked me already, so I am thinking of carrying it Will share more details later.

cheers

 

[kaushik]

Need help regarding DAC mobbing .

I have a pioneer Dv 300S DVD player.I feed 2 Towers fitted with amps [RCA -RCA analogue connection] .

yes it is good for video .. but the audio output is not satisfactory.I witness very poor mid and high end with this .[ although 24 bit DAC is specified...usual case in low cost players] .

I cant go for HiQ CDP because of cost .
Can any one suggest a DAC mobbing technique , that can be applied for
[ want to spend ... max 150$... ].

I have seen some kits for Audio CDP construction.... costing 500$+..for full player .But in my case changing DAC board will be enough i think.

 

[venkatcr]

Need help regarding DAC mobbing .

I have a pioneer Dv 300S DVD player.I feed 2 Towers fitted with amps [RCA -RCA analogue connection] .

yes it is good for video .. but the audio output is not satisfactory.I witness very poor mid and high end with this .[ although 24 bit DAC is specified...usual case in low cost players] .

I cant go for HiQ CDP because of cost .
Can any one suggest a DAC mobbing technique , that can be applied for
[ want to spend ... max 150$... ].

I have seen some kits for Audio CDP construction.... costing 500$+..for full player .But in my case changing DAC board will be enough i think.

I am not sure I understood you well. You want to change the DAC in your DVD Player? This is not something you can do at home. In most DVD Players, the DAC will be a chip that is integrated into the motherboard. You have to know the motherboard circuitry very well to bypass the circuitry and build your own DAC circuit. In the US, this is done by dedicated electronics experts who attempt with a few units, then get their design right. Also this is not done on all - only on some dedicated units the experts feel are worthwhile.

The better option will be to buy an external DAC system and route your output through that. At your price, the closest high quality DAC I can think of is the Beresford. This will cost you some 7,500 Rs.

BTW, the modification of any circuitry including a DAC is called Modding, not Mobbing.

Cheers

 

[kaushik]

thanx venkat,

i intended to add one more board internally tap the internal DAC inputs.... seems may not be feasible option.. as per your pointers ,

actually i have seen low jitter clock boards being sold, to add to existing CD Player for better sound ,because of that i thought extra DAC board also may be available !

i wl go the ext-DAC way, but doubt weather the cheap transport output[possibly!] limit external DAC utlity...

oh yes modding is correct word...

thanks a lott for the detailed reply ...!!!