Technical & Editorial
- Written by Dr. David A. Rich
- Published on 23 September 2013
- AVR - Audio Video Receiver - Build Quality: Part I
- Page 2: Understanding DAC Specifications
- Page 3: Digital Reconstruction Filter
- Page 4: Number of DACs per Chip
- Page 5: Improved Distortion and Noise Performance with Balanced DAC Output
- Page 6: Enhanced Distortion Performance with Current Mode DACs
- Page 7: Multiple DACs Combined to Produce a Single Channel
- Page 8: Chart Presenting Build DACs used in AVRs Across Manufacturers and Price
- Page 9: The Right Side of the Chart: More Details about the AVRs and Pre/Pros
- Page 10: The concept of Effective Bits
- Page 11: Single Chip Analog AVR LSI
- Page 12: Enhanced Performance with SSI Parts
- Page 13: Limitations of Operational Amplifier Performance with the Single Chip Analog AVR LSI
- Page 14: Limitations on the Performance of Semiconductor Switches with the Single-Chip Analog AVR LSI
- Page 15: Use of Relays to Achieve Better Performance
- Page 16: A Very Brief Look at Changes in Power Amps in AVRs
- Page 17: Conclusions
- All Pages
The Concept of Effective Bits
Signal-to-noise ratio (SNR) and Total Harmonic Distortion (THD) of the DAC are expressed in dB. This is the convention adopted by DAC manufacturers for presenting the metrics on datasheets. It is a challenge to decipher the relationship of these numbers relative to the requirements to reproduce music recorded in high resolution. It is nonetheless possible to convert the SNR to an equivalent specification called Effective Bits of Performance with minimal effort. As the name implies, effective bits indicates the limit of the performance of the DAC.
We can calculate the effective bits for both SNR or THD.
While high resolution music is recorded with a 24 bit word per sample, the ADCs and analog electronics used for the highest quality recordings are limited to the 20 – 21 bit range.
|Effective bits||SNR in dB|
When the number of bits of the word coming into the DAC is subtracted from the effective bits the remainder is often called the marketing bits. For example if a DAC that has 20 effective bits of signal to noise and has an input word length of 32 bits, it has 12 marketing bits.
Effective bits are also a useful measure for analog signal blocks. The SNR and THD specifications are converted with the same equation to clarify when the performance of the analog block will swamp the performance of the DAC. More specifically, this occurs when the effective bit specification of the analog part is lower than that of the DAC.
For a few data sheets on data converters, notably the ESS products, the number of bits in the data path, or multiply-accumulate section of the digital signal processor, are presented. The large number of extra bits, relative to the analog resolution of the DAC in effective bits, are required to prevent round-off error in the digital computations occurring in the on-chip digital reconstruction filter and Asynchronous Sampling Rate Converter.