Digital monitoring control with 82xx/72xx series - Forum

If I understand your problem properly, you want to use an AES/EBU source and an analogue source with your monitoring system. Unfortunately TC Electronic BMC-2 does not seem to have an analogue input, just three digital audio inputs. It seems to have a DA-converter function and you have the option of using digital audio or analog audio as the method for linking down to your monitor loudspeakers.

(a) Monitoring in mono using GLM

GLM software can use System Setup files. Each System Setup file defines the monitor system cabling and acoustical calibration. You can use System Setup files to force selection certain input signals. This means you can create a system setup file for a stereo pair of speakers where both speakers are listening to the A and B channels in the AES/EBU audio. This puts your stereo pair into a Mono mode. Normally, your left speaker is reproducing one of the channels, say A, and your right speaker is reproducing the other, say B, and you get stereo reproduction. The subwoofer is listening to several audio channels all the time, so no changes are usually needed there. So, to switch from stereo to mono reproduction using GLM, you create to System Setup files, one for mono and one for stereo. This can be easily done by taking a copy of your existing System Setup file, and editing in the copy the channel settings manually to turn on reproduction for both A and B channels for both loudspeakers. This method can be used for AES/EBU signals.

(b) Optimizing the processing chain for analog source monitoring in the stand-alone mode (without computer and GLM).

To optimize the signal processing chain for signal quality in stand-alone mode and using analog signals, it is usually a good idea to drive the possible AD converter using an analog signal as high as possible to keep you safe headroom for clipping. When the Genelec DSP loudspeakers are operated in the stand-alone mode (without GLM) the volume setting defaults to 0 dB FS (full open) after a loudspeaker re-start. This usually results in signal level that is too high. Then, you want to bring the speaker back on the GLM control network, and go into each loudspeaker's settings in GLM, and edit down the Level setting in the Acoustical Editor. Usually good analog system calibration is found when the input sensitivity is decreased by about 10 dB from the value set by GLM AutoCal. Do this for all loudspeakers and subwoofer(s). For each loudspeaker and subwoofer, save the settings in the loudspeaker/subwoofer for stand-alone operation. Now remove GLM and restart the loudspeaker system. Now you are in stand-alone mode (working without computer and GLM) and you are able to use conventional analog signal level control to adjust the monitoring level.

(c) Switching between separate AES/EBU sources

GLM can provide volume, dim, mute, and mono, but not source switching. Unfortunately Genelec loudspeakers do not provide multiple parallel AES/EBU input or the possibility to switch between them. However there are signal cross switch routers available on the market that can also be used for AES/EBU, and these can provide the ability to select between several AES/EBU signals. When you are using analog signals and you have a DSP subwoofer in the system, you need an AD converter (for example Genelec AD9200) before the subwoofer.

(d) Volume control in 8200/7200 vs. at the source

The volume can be controlled at the signal source (Protools, BMC-2, or some other source). It is always advantageous to have the volume control as the final action before the signal hits the drivers in a loudspeaker. This is why it is a good idea to use the volume control provided in the Genelec DSP loudspeakers and GLM. This is particularly useful if your source audio is using 16 bit word length. One good reason for this is that you may not know exactly how well your source is implementing the level control function. I have to point out that for most professional audio processing gear, the implementations are good, and no major problems should be expected. However, it is still a good idea to save the level control as the last action before the audio hits the drivers.

(e) Several AD-DA conversions

AD and DA conversions are always noisier than pure digital audio signal processing. Several cascaded AD-DA conversion stages should always be avoided if possible. AD and DA technology simply is not as quiet as DSP even if significant progress has been made over the last several years.

Your questions are relevant and important. You are doing the right thing optimizing the performance of your signal monitoring chain for the best performance.

GENELEC
Aki Mäkivirta
R&D Manager, DSP Products