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RACAL AM1601 Audio Multiplexer



There’s increasing interest in the use of audio recording as an adjunct to video surveillance, particularly on multi-camera systems. However, advances in video multiplexing -- which enables the outputs from up to 16 cameras to be recorded on a single time-lapse VCR -- have not been matched by similar developments in audio recording technology. The relatively small number of multi-channel audio recorders on the market tend to be cumbersome and expensive devices, making them unsuitable for routine or cost-sensitive installations.


The case for recording sound as well as a picture is compelling, and a growing number of cameras are now fitted with microphones, or have them available as an option. There are also circumstances where it would be useful to record sound from more than one microphone, associated with a single camera. Sound is a valuable source of information, and it’s a powerful evidential tool in its own right, but there are considerable difficulties involved in recording more than one or two channels of audio at a time. Most time-lapse VCRs can only record a single soundtrack, and then only on the shorter 3, 12 and 24 hour modes.


The Racal AM1601 provides a partial solution. It works on broadly the same principles as a video multiplexer, up to 16 channels of sound can be recorded simultaneously on a modified stereo VHS video recorder. More about that in a moment. An 8-channel multiplexer is also available, the AM0801 works in precisely the same way and -- apart from the number of channels -- has an identical specification.


The multiplexer is built inside a slim rack-mountable case measuring 42 x 444 x 309mm. Most of the front panel is taken up by a row of 16 numbered buttons, one for each input. The buttons are surmounted by pairs of green and red LED indicators. The green LED shows the input has been selected, the red one flickers in response to audio activity on the channel. On the right side of the fascia there’s another button for selecting monitor or replay mode, and the mains on/off switch.


On the back panel there’s a row of 16 phono (aka RCA and cinch), sockets, one for each line-level audio input (it cannot handle high-impedance microphone inputs); three further phono sockets for the input and output connections to the recording VCR, plus a line-audio output, to an amplified loudspeaker or monitor. The two remaining sockets are a 9-pin D-sub socket, for an external RS-232 control interface, and a standard IEC mains socket. The RS-232 link enables a limited number of the multiplexer’s functions to be monitored and controlled by a PC, either directly or remotely over a phone line, using a modem.


The system is designed to work with stereo hi-fi VCRs, that have a DFM (depth frequency multiplex) recording system. The frequency response of an ordinary audio cassette recorder or a standard monaural VHS soundtrack is insufficient for this application.


Video recorders used with the multiplexer require additional circuitry for the interface and signal processing. A VCR modification board (VMB) fits inside the machine and is connected to the video recorder’s audio recording and replay circuitry. Racal suggest the VMB can be used with most stereo VHS or S-VHS video recorders, where head-switching is carried out in the pre-amplifier. Reasonably comprehensive installation instructions are supplied with the VMB, that should provide enough information for most competent engineers. (A copy of the VCR’s service manual and an oscilloscope are also required). However, it’s likely that most installers and end-users will opt for a factory modified VCR. Our review sample came with a Mitsubishi HS-551 VCR; Racal tell us this is now a standard accessory.  



The brief instruction book is reasonably easy to follow, it provides a useful insight into how the system works, along with all necessary installation information.


Inside the multiplexer each line-level audio input is sampled in sequence, the information is then assembled into a TDM (time division multiplex) ‘frame’.  Each frame is preceded by a synchronisation pulse, this allows the multiplexer to reconstitute the separate signals during replay. The output signals, containing the sampled audio channels, then pass through an FM modulator, before being fed to the dedicated input socket on the recording VCR. Each audio channel has a bandwidth of approximately 200Hz to 3.5 kHz, which in general audio terms is ‘lo-fi’, but it’s good enough for speech and incidental sounds.


At switch-on the multiplexer defaults to monitor mode, with all of the channel inputs deselected; the red activity LEDs show if there’s any sound on the connected channels. Each channel has to be manually selected, which isn’t a problem if they’re all in use, but it seems something of an oversight not to have included some form of memory or back-up, that would automatically re-select used channels in the event of a power failure.


During normal operation selected channels are automatically routed to the multiplexer’s FM output. They’re also mixed together on the unit’s audio output; unwanted channels have to be manually disengaged to monitor a single channel. To monitor playback of a recording the procedure is the same; the red LED indicators show which channels are active. They’re chosen by the selector buttons, the sound is routed to the audio output socket. This is a line-level signal that in order to be heard has to be amplified. A small on-board audio amplifier, with speaker output terminals would have made installation a lot easier. As it stands the unit has to be used with an active speaker (with a built-in amplifier), a separate amplifier and speaker could also be used, or the output could be routed into the audio input of a video monitor, all of which adds to the complexity of the set-up.



Audio recording quality is reasonably good considering the narrowness of the available bandwidth. There’s a fair amount of background hiss but that’s only apparent when ambient noise levels are low. Switching is noiseless, and there’s no apparent crosstalk between channels at normal listening levels.   



Technically the AM1601 works well enough but it raises a number of issues concerning the practicalities of multiplexed audio. Firstly the specifics: the unit would definitely have benefited from an amplified audio output and some sort of memory backup for the channel selector. The monitor system is far from ideal. Listening to a single channel means de-selecting all of the others, which means they’re not being recorded. This raises the possibility that the operator might forget to re-select all of the channels afterwards.


Whilst we’re on the subject, other features that would improve operation include a sequencing facility, so it could be linked to a video multiplexer or switcher. Recording times could be increased dramatically (and operator workload reduced) with an alarm recording/triggering function, though clearly this would entail further modifications to the VCR. There’s no facility to time or date-stamp recordings, nor any way to link them with recorded video, which limits their usefulness as evidence.


In more general terms, it’s worth pointing out that the maximum recording time is just 3 hours, (6 hours is possible on stereo VCR’s that have an LP recording mode). That’s insufficient for unattended overnight recording, let alone provide coverage for weekends. Short recording times limit its use to manned installations, requiring the operator to either regularly change tapes, or manually monitor activity across the channels.


Audio multiplexing clearly has a lot of potential but this implementation fails to address the main problem, namely short recording times of just a few hours. Unless and until cost-effective systems are developed,  with recording times comparable with time-lapse VCRs, it cannot have the same kind of impact video multiplexing has had on the surveillance market.



Design and design features              ***

Circuitry and components                  ****

Ease of installation and wiring    ****   

Range and variety of functions            **     

Accompanying instructions                   ****                             

Technical advice and backup            ****     

Value for money                                                    




Ó R. Maybury 1996 0906











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