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My system-on-chip already has an onboard codec. So why should I bother with using external converters?
As an audio marketing manager, I get asked this question by many different customers. When we look at high volume consumer products like televisions, set top boxes and DVD/Blueray players, many system-on-chip (SOC) devices usually have integrated converters. On the surface of things, this looks like a great idea. Everything on one chip! However, not all is as rosy as it may look on the outside.
One of the biggest challenges semiconductor manufacturers face is balancing features and performance with the silicon process within which we make the products. Many of you may have heard of things like 90-nm processes, 45-nm processes, etc. These are all examples of the silicon processes within which semiconductor manufacturers try to make their new products. Each manufacturer has its own processes; each with its own advantages.
With the push to get more digital processing onto one small low-cost piece of silicon, the focus for all process designers is to push the digital transistor size even smaller. It does seem, however, that as we make the process smaller and tighter, the system's analog performance begins to suffer.
Making ADCs and DACs that exceed 96 dB of dynamic range is getting harder and harder in the small geometry silicon that many ASICs are made in. Consider a high-performance set top box (STB) processor. Inside a modern STB chip is a real-time video decompression processor, a microprocessor for the system, Ethernet subsystems, hard drive interfaces, and lots of other features. The majority of these blocks are digitally-based, and each benefit from the shrinking of the silicon process.
Mixed signal and analog system blocks don't always fare so well. As digital processes begin to focus on speed and size, the processor's real analog performance becomes a secondary concern. To many, digital is just overdriven analog with a very low signal-to-noise ratio (SNR)! What this means in the real world is that getting higher quality audio performance from a SOC is becoming quite difficult. Many SOCs manage around 90 dB+ dynamic range, but mostly by using a differential output to squeeze the extra 3 dB of performance.
In a cell phone system, 90 dB can be considered to be relatively good quality, however, in today's home entertainment environment 90 dB is seen as ancient technology. Modern AV systems typically provide a minimum of 105 dB of performance, with better systems specifying up to 120 dB of performance or higher.
Moving to external, higher performance audio converters does have its benefits in a home and portable environment. In a low-grade 96-dB DAC, you need each of the bits in your 16-bit world working at full scale to maximize the difference between your fixed quantization noise and your audio output.
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