Design Article
Why Use a Better DAC?
Dafydd Roche, Texas Instruments
12/3/2008 2:11 PM EST
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.
Comments
Guru of Grounding
12/6/2008 6:23 PM EST
The author has confused the issue of differing level standards between professional and consumer audio equipment. First, the nominal or "reference level" signals are +4 dBu (1.23 volts rms) in pro gear and -10 dBV (0.316 volts rms) in consumer gear. The 2.0 volt rms or 5.6 volt pk-pk number for consumer gear refers to a full-scale rating. This level is only 16 dB higher than the consumer reference by the way, not 22 dB. Therefore, a consumer output is still legitimately rated at -10 dBV reference level but will clip at an output level 16 dB higher - this is referred to as "headroom". Professional systems operate at levels 12 dB higher than this, so they must be capable of passing signals up to +20 dBu (7.75 volts rms or 22 volts pk-pk) without clipping to preserve 16 dB of headroom. The main difference between "pro" and "consumer" or "MI" (musical instrument) gear is the choice of interface. Unbalanced interfaces are suitable only for very short cables. Common-impedance coupling is inherent to unbalanced interfaces and is its downfall in high-performance audio systems. Only gear with balanced interfaces is worthy of the term "professional". However, modern "pro" interfaces based on simple diff-amps have tarnished the reputation of balanced interfaces unfairly - but that's another story - see http://www.jensentransformers.com/an/ingenaes.pdf for the how and why. Then you'll realize why the IEC changed the test procedure for "balanced inputs" back in 2000. --- Bill Whitlock, president and chief engineer, Jensen Transformers, Inc., AES Fellow and IEEE Senior Member.
Sign in to Reply
Guru of Grounding
12/10/2008 1:35 PM EST
The author has confused the issue of differing level standards between professional and consumer audio equipment. First, the NOMINAL or "reference level" signals are +4 dBu (1.23 volts rms) in pro gear and -10 dBV (0.316 volts rms) in consumer gear. The 2.0 volt rms or 5.6 volt pk-pk number for consumer gear refers to a FULL-SCALE rating. This level is only 16 dB higher than the consumer reference by the way, not 22 dB. Therefore, a consumer output is still legitimately rated at -10 dBV reference level but will clip at an output level 16 dB higher - this is referred to as "headroom". How much headroom is enough depends on the program material, but typically ranges from 12 dB to 20 dB. Professional systems operate at reference/nominal levels 12 dB higher than this, so they must be capable of passing signals up to +20 dBu (7.75 volts rms or 22 volts pk-pk) without clipping to preserve 16 dB of headroom. The main difference between "pro" and "consumer" or "MI" (musical instrument) gear is the choice of interface. Unbalanced interfaces are suitable only for very short cables. Common-impedance coupling is inherent to unbalanced interfaces and is its downfall in high-performance audio systems. Only gear with balanced interfaces is worthy of the term "professional". However, modern "pro" interfaces based on simple diff-amps have tarnished the reputation of balanced interfaces unfairly - but that's another story - see http://www.jensentransformers.com/an/ingenaes.pdf for the how and why. Then you'll realize why the IEC changed the test procedure for "balanced inputs" back in 2000. --- Bill Whitlock, president and chief engineer, Jensen Transformers, Inc., AES Fellow and IEEE Senior Member.
Sign in to Reply