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.

Choosing the DAC, Part 1 -- Basic Theory

When we talk about digital to analog conversion in our application, we are referring to converting PCM (pulse-code modulation) data into an analog signal. This resulting analog signal can either be represented as an electrical voltage or current. So, for a voltage output, a number is the input and a voltage is the output. These numbers are updated at the sampling interval (or sampling rate) of x times per second. For example, CD data is represented with a 16-bit number at a sampling rate of 44,100 times per second. So a 16-bit number, which represents the amplitude of the audio at a point in time is sent to the DAC (digital to analog converter), along with a clock signal, which causes each number to be stored with the correct sequence and timing (lets ignore that relatively important detail for now). Then the output voltage changes very rapidly from the previous value to the value represented by the currently stored number. So basically, the voltage (or current) is set and held in time until the next number is set. What you end up with is a bunch of rectangular pulses that look like the red line shown in the graph below, instead of the smooth gray lines which represent the ideal analog signal.

The differences between the red stair steps and the smooth waveform create a type of distortion in the signal called aliasing. According to good ole Harry Nyquist, the aliasing distortion can be avoided if the sampling frequency is two times greater than the bandwidth of the maximum frequency of your signal. That may not make a lot of sense, so let me use CD audio in another example. Many experts believe the frequency range of human hearing is between 20 and 20,000 Hz. For the majority of us, we can’t hear any sounds above or below that range. So, if you apply the Nyquist sampling theorem (actually Vladimir Kotelnikov and Claude Shannon deserve some mention here, but our friend Harry seems to get all the credit) and double the upper range, we get a sampling rate of 40,000 Hz. So to avoid the aliasing distortion, 44,100 Hz was selected as the sampling frequency for audio CDs.

Welcome to the End

Nice blog post title... I bet some of you thought I was announcing the end of Ampliozone. I'm not. I hope to keep this going for a long time. Actually, this week's SongSpot is a song called 'Welcome to the End' off the album The Universe is Expanding from the synth-pop band Anything Box. The group was formed in New Jersey in 1986 by Claude S. with his friends Dania Morales and Paul Rijnders. Their name comes from the title of a book of short stories. A lot of their work is described as soothing and melodic. One of their albums, Worth, is described as the best album Anything Box ever made, but unfortunately was never released commercially. However, it looks like you can buy it at CD Baby.

All About Sound, Part 12

This is it. The final installment in the introduction to sound technology from Scott Janus' book "Audio in the 21st Century" (courtesy of Audio DesignLine).

In Part 12 he writes about Measuring Sound Quality, Harmonic Distortion, Total Harmonic Distortion, and Signal to Noise Ratio. I'll probably want to cover some of these topics in a little more detail in future posts and discuss how these measurements are applied to the technology in our products.

I hope you enjoyed this series.

All About Sound, Part 11

The introduction to sound technology from Scott Janus' book "Audio in the 21st Century" (courtesy of Audio DesignLine) continues.

In Part 11 we learn more about Sine Waves, Triangular Waves, Square Waves, and Sawtooth Waves..

SongSpot for Elections

Tomorrow is election day here, so I chose a song that might inspire people to go out and vote (or not). This weeks SongSpot is called 'Breaking Through' from the album Long Way from Tomorrow by the band Small Change Romeos.

CES 2007

Are you going to the Consumer Electronics Show in Vegas this year?

I was planning to go, but I can't find a reasonably priced room somewhere near the strip. I attended CES 2006, but had to stay at the Howard Johnson's near the airport. It was cheap, but not necessarily the greatest accommodations. Given that we are a startup with no income, cheap is very good. Anyhow, even the HoJos is all booked up from January 7th to the 11th, 2007. I'd also like to find something a little closer, so I don't have to rent a car.

So, if you know of any alternatives, please let me know.

All About Sound, Part 10

More audio theory...

Part 10 from Scott Janus' book "Audio in the 21st Century" courtesy of Audio DesignLine introduces us to the frequency domain.

All About Sound, Parts 8 and 9

There's more!! Part 8 from Scott Janus' book "Audio in the 21st Century" courtesy of Audio DesignLine continues to cover sound pressure and intensity. Part 9 uses a few examples to explain sound pressure and decibels in more detail.