Use with Digital Sources
- DA converter is the limiting factor
- all bits in DA have to be used
- scale signal down in analog domain
- maximize signal in digital domain for monitoring
- match max output of DA to dynamic range of the monitor speaker
The limiting factor in the modern all-digital studio is the DA conversion at the monitor loudspeaker. The Genelec loudspeakers have a dynamic range capability that is larger than the dynamic range available from any DA converter today. The DA conversion has to be carefully set and adjusted to make sure that all details in the high dynamic range digital signal become audible. Misuse of the DA conversion may lead the engineer to blame the monitoring system for low quality as the details of the signal get embedded into DA converter noise, and the quantization distortions may become audible. This paper describes guidelines to set and operate digital sound processing and reproduction systems properly with Genelec active monitoring speakers.
Dynamic rangeMore and more of the studio signal processing is turning digital. Digital mixing consoles are becoming increasingly popular. Then, the ultimate borderline between digital and analog worlds is at the interface from the mixing console to the monitoring loudspeaker. There, the digital audio signal will finally be turned into an analog signal to be reproduced.
The digital-to-analog converter becomes the limiting factor to convey the quality of the fully digital system into high quality studio monitor speakers. To solve the problem of interfacing digital mixing consoles to active monitoring loudspeakers, we must first consider the basic differences between the analog and the digital signals.
For an analog signal, there is a practical minimum and a maximum value. The low limit is the noise level in the audio system; the high limit is the distortion or clipping level of the system. In some analog systems there is no significant distortion before the clipping occurs. In others, there is a slow increase in distortion that limits the useful signal magnitude.
The digital signal is represented by numbers. Usually the signal is represented with fixed point numbers. This means that we can not represent fractions of values. Therefore, there is a very distinct minimum value to a signal, as well as a distinct maximum value. It is not possible to have any values outside of these ranges.
Practical exampleThe concept of the dynamic range is fundamental as the digital world meets with the analog. The dynamic range is the range of useful values of the signal representation. For an analog signal it is determined by the noise level and the distortion or clipping. For a digital signal it is determined by the largest and the smallest value of the signal we can represent.
Lets take a practical example. A typical two-way active speaker by Genelec has an input referred noise level of 3 uVrms in the most sensitive 3-5 kHz region. The maximum input signal is 1 Vrms This is determined by the clipping limit in the Genelec speaker, not by distortion limitations, as the Genelec products have been designed to provide maximum performance with digital sources. The dynamic range of this speaker becomes a comfortable 130 dB. This is enough to render the noise generated inside the loudspeakers amplifiers inaudible in all normal listening situations. The speaker can accept a signal that has a peak value of 1.4 V.
A digital studio may process audio with 24 bit resolution. As both positive and negative voltages need to be represented, only 23 bits can be used to represent the signal value. The smallest value is represented with one bit. If we use one bit we can represent two values (0 and 1). If we use two bits, we can represent four values (0, 1, 2 and 3). If we use 23 bits, we can represent 8388608 values. Therefore, we can process the signal with 138 dB digital dynamic range.
Digital-to-analog conversionThe problems really begin when we want to move from digits to voltages. This is done using the digital-to-analog converter (DA converter). As the DA converter represents an interface between the digital and analog worlds, it is plagued with both the problems of digital signal processing as well as difficulties of analog signal processing.
The DA converter has an analog noise level. This sets one limit to the lowest signal that can be usefully generated with a particular DA converter.
As we make the signal smaller, we use less and less bits. This is called digital attenuation (see Figure 1). When we represent a very small signal with only few bits, we can only represent few possible signal values. This is called quantization. As we quantize to a small number of bits we are making a rather gross approximation. This approximation produces distortion. And this distortion is very disturbing, if it happens to fall to such voltages where we can still hear the signal. High quality digital audio systems take special measures to reduce the audible effects of quantization.
We can never emphasize enough the central importance of the monitoring DA converters -- after all they are the main instruments you will use to decide on the quality of your audio material. As a rule, the DA converter should be of the highest quality because the problems in it will be reflected to everything.
Figure 1. The effect of digital attenuation.
Matching the dynamic rangesTo make most of our equipment, we would like to match the digital and the analog dynamic ranges in the best possible way.
We should match the maximum voltage that the DA converter can generate with the maximum voltage the active speaker can accept.
To do this we should first check what is the maximum voltage produced by the DA-converter when it is driven with a full scale signal -- at this point the digital input is said to be 0 dBfs. Use a 1000 Hz digital sinus signal to make this check. Such a signal is readily available on most test signal CD records. Turn off your monitor speakers, play the test CD and measure the peak value of the voltage present at the input to the monitor speakers using a precision voltage meter or a high quality multimeter. Lets say that this voltage on some digital mixing console happens to be +18 dBm, or 6.2 V.
We immediately see a problem. The speaker can only accept a 1.4 volt signal. The DA converter is supplying 6.2 volts. We loose 4.8 volts of the useful signal range. This will result in two things. Firstly, the noise level in the digital system appears to be higher than expected because we are mismatching the digital system dynamic range with the analog system dynamic range at the DA converter. Secondly, the audio signal will clip at signal levels exceeding the 1.4 volt limit.
To correct the clipping the audio engineer may take down the digital output level pot (Figure 2). This is not wise. The required attenuation in our example would be 13 dB. The noise voltage at the DA converter does not change, so it appears that the noise level is left some 13 dB higher than it should be. We have lost a total of 26 dB of the useful dynamic range because the output voltage range of the DA converter does not fall on the input voltage range of the monitor loudspeaker. We are operating with only about 74 dB of effective dynamic range, and the expensive digital console operates with the quality of a 13-bit system. We have lost 11 of the 24 bit resolution. This is clearly unacceptable!
Figure 2. Wrong gain matching in digital domain.
The right matchThe output voltage range of the digital mixing console should be matched with the active monitor speaker dynamic range by using analog attenuation or gain padding between the mixing console and the active monitor (see Figure 3).
If the output voltage is too high, then we will use a voltage divider network to reduce the input voltage to the active monitor speaker. This also reduces the bottom noise voltage of the DA converter simultaneously, maintaining the maximum dynamic range.
In practice several attenuations may be needed to optimize the DA converter dynamic range for all listening situations. There may be different settings for low and high output levels. Such an analog gain adjustments should actually reside inside a mixing console as a part of the monitoring output DA converter.
Figure 3. Correct gain matching with analog attenuator.
If the gain needs to be increased, we have an exceptional situation. First check that you have not missed any gain adjustment, set the output gain to maximum, and remeasure. If you are still reading smaller values than needed to obtain maximum output from the monitor speakers, then we need a very high quality gain stage. The noise level in this gain stage should be lower than the noise level of the DA converter and the monitor speaker. As this is extremely unlikely, please recheck again that you have not missed some adjustment in the DA converter.