Thursday, November 16, 2006

Choosing the DAC, Part 2 -- Narrowing it Down

We're still trying to pick the best DAC (digital to analog converter) for our product. We've looked at all the data sheets and compared the specs and test results. There are hundreds of DACs from several manufacturers to choose from. Using the specs, we were able to identify what we think are the best products on the market. Since we want to support playback of all currently known audio formats (at least those that you can play using an HTPC, which means no SACD because Sony/Philips will not license the necessary hardware and software for playing SACD with a PC), we will need to use a DAC that has a sampling rate of up to 192 kHz and bit depth of at least 24 bits. For our application, we would also like to use DACs with fully differential outputs because of the benefits of eliminating noise from the signal path and the fact that our amp modules work best with a differential input. That narrows our choice down a little bit.

The three key parameters that we looked at for evaluating different DACs were the signal-to-noise ratio (SNR), dynamic range (DR) and total harmonic distortion (THD+N). Based on the three main criteria, we decided to limit our final choice to a DAC with a SNR and DR of at least 120 dB, and a THD+N of at least -100 dB. Here’s a list of DACs that meet our specifications:

Manufacturer

Part #

SNR

DR (dB)

THD+N (dB)

AKM Semiconductor

AK4396

120

120

-100

Cirrus Logic

CS4398

120

120

-107

TI/Burr Brown

PCM1792

132

129

-108

Analog Devices

AD1955

120

120

-110


We've also tried to contact a representative of these highly respected manufacturers, Texas Instruments/Burr Brown, Cirrus Logic, Analog Devices, and AKM Semiconductor, for advice. So far the only ones to respond have been Richard Kulavik, Manager of Marketing and Applications at AKM and John Melanson of Cirrus Logic. TI/Burr Brown, and Analog Devices have yet to respond. Could it be that Amplio is just too small and insignificant? Maybe -- that's one of the challenges and frustrations of being a start-up.

4 comments:

  1. have a look at the wolfson codecs like the wm8770 as used by the terratec aureon 7.1 firewire soundcard. The wm8770 comes with an integrated analog volume control.

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  2. Thanks for the recommendation. I have looked at the Wolfson DACs, but they didn't make our final cut. The one you are recommending is actually a combination DAC and ADC, which is typically called a CODEC (for code/decode). The audio performance of the Wolfson WM8770 is also not at the level of those on our list. The WM8770 SNR is 106dB compared to 120dB and -96dB THD+N vs. -100 to -110dB for the others. We also prefer to use multiple stereo DACs instead of a single multichannel DAC. This will also give us better performance. Wolfson also sells the WM8740, which is their flagship or highest performing DAC. It still doesn't quite measure up to the performance of their competition. Wolfson does have a nice analog volume control chip, which is also much better than the integrated volume control in the WM8770. Our goal is to provide DAC performance on par with most of the relatively expensive dedicated stand alone DACs, so we need to spec fairly high end components.

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  3. What about Wolfson WM8728 ?

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  4. Diego -

    The Wolfson WM8728 doesn't meet our spec either. It has a dynamic range of only 106 dB and a THD of -97 dB (0.0014125%). Even though the distortion measurement is pretty good, it isn't as good as any of the others, including the TI/Burr Brown chips, which are -108 dB (0.0004%). We also considered the Wolfson WM8740, but again it didn't measure up. The WM8741 might be worth checking out as well. It has very good dynamic range (128 dB), but only good THD (-100 dB or 0.001%). Still not as impressive as the TI/Burr Brown PCM1792.

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