Audio specifications are one of those things that you see with every audio product but rarely do they reveal anything that gives you an idea what the product sounds like. This is unfortunate, because reliable measurements would save all of us a lot of time and effort as we try to pick the great products from the not-so-great.
The biggest problem is that as audio products have gotten better, our ability to measure them has been found wanting. That leaves all the published specifications about the same for all products. Here is a quote from the Audio Precision (the leader in audio measurement systems) web site about their best system:
"With a typical residual THD+N of -120 dB and over 1 MHz bandwidth, the APx555 surpasses the analog performance of all other audio analyzers"
Its a great system, and costs almost $30K, but not able to see all the things you need to see. So you can see that we have had to change the rules and be clever to measure DACs with lower noise. In the block diagrams below you can see two of the MSB test systems. They take advantage of the digital volume controls included in most high-end DACs to make a greater than 24 bit test signal and makes use of an incredibly low-noise amplifier built specifically for making these low-level measurements.
Effective Bits Measurement System
Out of Band Noise Measurement System
Custom Measurement Systems
So the key to making detailed and accurate measurements is to build a measurement system tailored for the specific measurement. These two low-noise systems were used to measure DAC chips, and the results matched the published specifications very well. They were used to measure competitors DACs and when compared to the published measurements, those DACs matched well and in most cases slightly exceeded the published values. This is because the commonly used test systems are near their noise limits. We even measured resistors and finally a dead short (included in the measurements) and all tests confirmed the validity of the system.