Finally, a Couple Things to Get Excited About

With the state of the economy and all, it's been kind of difficult to keep my spirits up. The nearly daily announcements of layoffs, stock market losses, etc., are a constant reminder of my own serious financial issues. In addition to these challenges, back in December I posted an article about the problem playing HD audio from Blu-ray titles because of PAP and the lack of any standards. I'm not really sure how to classify this problem, I guess it's more of a political or policy problem. There are also technical challenges that I haven't fully solved. For example, FireWire is a good solution for multichannel audio, but fewer and fewer systems are coming equipped with a FireWire connection. Anyhow, none of these factors make it easy to get this business off the ground.

Thankfully, there are a few things that give me optimism. There are two recent developments that really excite and/or interest me. One is the new 32 bit DAC from ESS Technology. The other is the soon to be released switch mode power supplies from Hypex Electronics. I don't have a lot of details to share here, but I can provide a basic overview and will try to get more information and write about it in a future post.

I've written a couple of articles comparing DACs and based upon the specs, it looked like the TI/Burr Brown DACs were best, but ESS actually exceeded all of these with their 24 bit DAC, the ES9008 Sabre. ESS officially announced their new 32 bit DAC around January 5th and I believe demonstrated it at CES 2009 in a private suite by appointment only. The press release claims that the new Sabre³² Reference DAC is the world's best 32-bit audio D/A converter. Using an enhanced 32-bit ESS patented Hyperstream architecture, the Sabre³² further extends the original Sabre Reference's from 24-bit to 32-bit music and raises its mindboggling performance even higher to 135dB dynamic range and -120dB THD. There are 2 versions of the Sabre³² Referernce DAC - the ES9018 8-channel DAC, which is the one we are most interested in, and the ES9012 2-channel DAC. The ES9018 8-channel DAC supports mono, stereo, 4- and 8-channel output modes. These DACs also support very good quality volume control, customizable filter characteristics and advanced jitter elimination capability. To be fair, I should also mention that ESS doesn't claim to have come out with the first 32-bit DAC. I think that achievement belongs to AKM Semiconductor. On December 9th, they announced their 32-bit AK4390 DAC, which is a very nice addition to their line of high quality DACs. However, around the same time as ESS's announcement of their new Sabre³² Reference DAC, they also announced their new Sabre³² Reference ADC, which is the first 32-bit audio A/D converter with an amazing THD of -120dB. Very cool stuff...

Another technical challenge we've been dealing with is the size, weight and cost of high quality linear power supplies. To provide power for 8 channels of amplification requires are very large and heavy torroidal transformer along with the fairly large and expensive capacitors in the power supply circuitry. This becomes a really big problem if you are using 400W or greater amp modules. That is why we've been very interested in the development of audiophile grade switch mode power supplies (SMPS). A lot of electronic devices today, like medical instruments and personal computers, use SMPS technology because of the size, weight and cost savings. However, there are very few choices for affordable SMPSs for audio applications. The biggest problem with a conventional SMPS is electromagnet interference (EMI) and the ability to deal with big changes in load current due to the audio signal amplification. It's easy to design a high quality linear supply for audio applications, but it's a lot more complicated with SMPS. Hypex has been working on this for quite a long time. Awhile ago, I think back in June of '08, they announced the availablity of their SMPS180. Just yesterday, I came across the data sheet for their SMPS400, which I think is suppose to be available in 6 weeks. I haven't had the chance to work with any of these, so I don't know how they sound. Hypex is a pretty low-key company. They don't make products to sell directly to consumers, so you don't get all the usual marketing hype. So it's hard to tell if their SMPSs are something they've made just to satisfy the demand from their OEM customers, or if they are really good. Since it's taken them a long time to develop these, I suspect they are pretty good. The engineers working at Hypex are really dedicated to very high quality audio, so I doubt they would put their names on something that doesn't match the high quality of their amp modules. Here's some useful information from their spec sheet:

"The SMPS400 is a high efficiency Safety Class 2 switch mode power supply specifically designed for use with our range of UcD^TM amplifier modules. Key features are high efficiency over the entire load range, extremely small form factor, low weight and very low radiated and conducted EMI. The SMPS400 also features an advanced overcurrent protection which in case of temporary overload simply reduces the output voltage, only when the overload condition remains for a longer time the supply will enter hiccup mode until the overload condition disappears. This feature combined with large electrolytic buffer capacitors leads to the capability of delivering high dynamic headroom power to the connected amplifier. The SMPS400 is optimized from the first phase of design to final implementation to realize the low EMI signature required of the most demanding audio applications."

"Conventional Switch Mode Power Supplies are commonly unsuitable for audio purposes due to poor peak power capabilities and the inability to handle reversed currents generated by Class D amplifiers as a load. The Hypex SMPS400 achieves these things by using an advanced over current protection circuit, a highly efficient 2 quadrant DC/DC converter which is capable of handling reversed currents and has a peak power handling of many times its rated power."

Like I said, I'm excited about this stuff and when I get more info and possible reviews from people who've used these new components, I'll write about it in a future post.

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.