Table 1. Open load SNR at dynamic microphone output. Click here to see an enlarged diagram.
Analog audio signals are affected by noise. Noise is best defined as an unwanted disturbance superimposed on a useful signal. The noise level is usually expressed in dB relative to a reference value and is commonly referred to as signal-to-noise-ratio (SNR). In professional studio equipment, the reference level for SNR measurements is the maximum output level (MOL) or 10dB above standard operating level (SOL). In a studio environment, there are two types of noise: random noise and coherent noise (hum and crosstalk). In this article, we will discuss random audio noise and its effects.
The main source of random noise is the thermal agitation of electrons. Given R, the resistive component of an impedance Z, the mean square value of the thermal noise voltage is given by En2 = 4kTBR, where
En = The RMS noise voltage
k = Boltzmann's constant
(1.38 × 10-23 joules/kelvin)
T = The absolute
temperature in kelvin
B = The bandwidth in Hz
T is usually assigned a value such that 1.38T = 400, corresponding to about 17° C. So En2 = 1.6 × 10-20 BR.
The SNR at the output of a system depends on the noise generated by the resistive component of the signal source — for example, the microphone. Assuming B = 20kHz and a microphone with a resistive component R = 150Ω, then En = 0.219µV. This is the theoretical thermal noise of the microphone input circuit. The microphone preamplifier contributes its own random noise, which considerably reduces the SNR of the system. The situation can be visualized as having an ideal noiseless amplifier whose input is fed by a noise generator. This fictitious noise is called the equivalent input noise (EIN) of the amplifier. The difference between the EIN and the calculated theoretical thermal noise level of the audio signal source is called the noise factor of the amplifier.
The measurement of SNR begins with a rather involved procedure, and the accuracy of the results depends on a strict adherence to a set of rules. The following routine test procedure is suitable for the SNR measurements of an audio mixer:
Step 1: Disable all inputs except the one in the measurement path. Disable all compressors and equalizers. Feed a 1kHz audio signal at the rated input level (e.g., -70dBu) at the microphone input and adjust input sensitivity, channel gain and master gain for SOL at the output (+8dBu or +4dBu).
Step 2: Remove the input signal source and substitute with a low-noise 150Ω resistor. Measure the noise at the output with the audio analyzer in dBu in a 20kHz bandwidth. An optional noise-weighting network may be used to simulate the ear frequency response.
The concept of dynamic range
The actual SNR is given by the difference, in dB, between MOL in dBu and the measured noise in dBu. The use of a weighting network may produce SNR values that differ by 10dB or more from flat 20kHz bandwidth measurements.
The overload level, also called MOL, is usually defined in terms of acceptable total harmonic distortion (THD). Although there is no universal agreement on the maximum accepted value for THD, the figure of 1 percent is generally quoted for audio consoles and distribution amplifiers. Analog audio equipment is adjusted such that the MOL is higher than the SOL or line-up level. The difference between MOL and SOL, expressed in dB, is called headroom.
|SPL (dB)||Microphone output voltage (µV)||SNR (dB)|
The MOL of an audio console or audio distribution amplifier is usually specified as 10dB or more above the SOL. Higher values of headroom may be needed when VU meters are used for audio signal level monitoring due to the meter masking effects.
Audio mixing consoles are also specified in terms of maximum input level (MIL). The MIL of an audio mixing console is the microphone input level at which the THD, due to the microphone input preamplifier, is 1 percent. The input headroom of an audio mixing console is the difference between the MIL and the rated input level (e.g., -60dBu). Input headroom specifications for audio consoles are between 20dB and 35dB.
Figure 1. Factors contributing to the dynamic range in a studio environment. Click here to see an enlarged diagram.
The minimum acceptable signal level in a system is closely related to the acceptable SNR at low signal levels. This is clearly an operational decision. Ideally, the SNR at the lowest acceptable signal level should not be lower than 40dB. Microphone sensitivity ratings, measured at 74dB sound pressure level (SPL), are commonly expressed in open-load microvolts or dBV (decibels with respect to 1V). Impedances of professional-quality microphones are standardized at 150Ω, but other values are also encountered in practice.
A typical moving-coil microphone, with a source impedance of 150Ω, generates an open-load voltage of 100µV (-80dBV) at 74 SPL. The input impedance of the microphone preamplifier bridges the microphone output — that is, it has a value of 1500Ω or higher, to avoid microphone damping and input signal-to-noise degradation due to excessive signal loss.
Table 1 on page 22 lists the theoretical SNR at a standard dynamic microphone output for several SPLs under open load conditions. This table shows that 40dB SNR can be achieved at an SPL of 61dB, assuming that the ambient noise in the studio is 0dB SPL. A higher level of ambient noise will raise the minimum acceptable signal level.
The dynamic range is defined as the difference, expressed in dB, between MOL and the minimum acceptable signal level. Figure 1 on page 22 shows how three basic elements, namely the microphone, the studio and the analog audio mixer, each contribute to a reduced dynamic range in a studio. There are a number of assumptions made here as follows:
- The microphone source resistance is 150Ω.
- The microphone sensitivity is -80dbV at 74 SPL.
- The recording studio ambient noise is 30dB SPL.
- The peak SPL is 120dB.
- The studio operates in a voltage-matching mode.
- The SOL is +8dBu.
- The audio mixer MOL at 1 percent THD is +18dBu.
- The audio mixer SNR is 80dB with respect to MOL.
- The audio mixer is lined up such that an SPL of 120dB produces MOL at the output.
The assumptions made above reflect the expected single-pass performance of typical equipment available on the market. These are ideal operating conditions. In actual practice, the results may be different and, possibly, worse.
Given SPL peaks of 120db, an ambient acoustical studio noise level of 0dB SPL and a standard dynamic microphone as defined above, the theoretical dynamic range at the microphone output is 59dB. It is limited mostly by the thermal noise of its resistive component of the mic.
The ambient noise in a broadcast studio is of the order of 30dB SPL. This limits the dynamic range of the studio to 50dB. Unlike the random noise generated by the resistive component of the microphone, the studio acoustical ambient noise has mostly low-frequency spectral noise components. Top-of-the-line analog audio mixers have an SNR on the order of 80dB with respect to the reference MOL. If the mixer is adjusted such that SPL peaks of 120dB generate output-level peaks (MOL) of +18dBu (10dB above an SOL of +8dBu) at 1 percent THD with an SNR of 80dB, the mixer background noise level is -62dBu as measured at its output. This limits the dynamic range of the audio mixer to 40dB.
In order to avoid mixer input and output overloading at high SPL levels and a reduced SNR at low SPL levels, the operator must ride the gain, meaning manually adjusting input signal levels, channel gains and the master faders to achieve optimum operating conditions. Some mixing audio consoles with a large number of microphone inputs feature individual input channel compressors to ease the task of the operator.
Michael Robin, fellow of the SMPTE and former engineer with the Canadian Broadcasting Corp.'s engineering headquarters, is an independent broadcast consultant located in Montreal. He is co-author of Digital Television Fundamentals, published by McGraw-Hill and translated into Chinese and Japanese.
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