原创 【音频测试基础知识】信噪比与动态范围

Signal-Noise and Dynamic Range

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Introduction

Hi-fi equipment specifications tend to include the terms ‘signal to noise ratio’ and ‘dynamic range’, both of which are confusing and best avoided. Noise has to be measured with reference to something, but the only sensible reference point is ‘alignment level’ (see article on this). Signal to noise ratio has no real meaning as audio signals are constantly changing so there is no such thing as ‘signal level’. Dynamic range is an ambiguous term that is commonly used in three different ways. To audio professionals it refers to the ratio of maximum to minimum levels in a recording or programme. It can also mean the difference between maximum permitted level (clipping level or full-scale digital) and noise level, but maximum level is often hard to define, for example on analog tape recordings, and the term has become corrupted by a tendency to refer to the dynamic range of CD players as meaning the noise level on a blank recording with no dither, in other words just the analog noise content at the output. This is not particularly useful; especially since many CD players incorporate automatic muting in the absence of signal to make them appear even quieter!

Subjectively Valid Noise Measurement

Professionals measure noise in dB below alignment level, which is a reference point above which ‘headroom’ exists up to maximum permitted level. Professionals often allow 18dB of headroom, as recommended by the EBU (European Broadcasting Union), so a noise level of –60dB ITU-R 468 would represent a dynamic range of 78dB, which if measured A-weighted might come out 11dB better at 89dB. A noise level of -60dB AL would be considered reasonably good by professionals, with –68dB representing the best attainable from 16-bit digital audio (noise shaped), and more than good enough for most purposes (see article 'Analysing Programme Levels').

The 96dB Myth

Audiophiles may talk in terms of 96 to 120dB dynamic range, but they often fail to refer to any measurement standard, making the figures meaningless. Attempts to calculate the dynamic range of digital audio on the basis that 16 bits represents a ratio of 65000:1 or 96dB are invalidated by the fact that the full digital count represents the peak possible level, rather than the rms equivalent of the maximum possible sinewave, while the minimum count of one has little to do with the noise level, which depends on the type of dither (or noise-shaping) used. They also fail to take any account of weighting for subjective validity.