Technical & Editorial

AVR - Audio Video Receiver - Build Quality: Part I


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 bitsSNR in dB
14 86
15 92
16 98
17 104
18 110
19 116
20 122
21 128
22 134

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.