Q: I have a system installed where audio is integrated with the on-site CCTV, which has a remote CCTV link via telephone lines. The audio system comprises of vandal proof call boxes, including a speaker and microphone. The audio quality both locally and remotely has always been "wooly". How can I improve it?
A: Most of the audio problems I have seen come down to impedance mismatches between equipment, or poor levels of signal feeding equipment which is again a mismatch. It may be worth briefly covering a little audio theory to best answer your question….
Like CCTV signals, audio signals should be within a specific voltage range to get the best results from the audio equipment; too low a level and the system will never be able to reach a reasonable volume level without large amounts of background noise; too high a level and distortion will result, certainly at higher volume levels.
Audio signals can also be balanced or unbalanced. The unbalanced type of equipment uses a 2 core connection with ground (screen) as part of the signal conducting circuit. Unbalanced signals cannot travel too far without excess interference to the audio signal and are also subject to earth loops. Balanced audio signals use a three core connection using a balanced pair of signals, often referred to as hot and cold. The screen is purely a protective ground and is not part of the audio circuit. Balanced audio signals can travel long distances; up to hundreds of metres with little effect to the signal. They are also immune to earth loops when correctly installed.
Additionally, there are also two commonly found levels of audio signal: microphone level which is normally within the millivolts range, and line level which is a standard 1 volt rms. Microphone level signals can only run for very short distances without ill effects, whereas line level signals can run over distances with fewer ill effects.
As far as input impedances go there are really two standards: high impedance for unbalanced signals which is usually measured in kilohms. Balanced signals are usually run into a far lower impedance and are measured in hundreds of ohms, typically 600 ohms.
From your description of "wooly" audio it sounds like the signal level from your microphone is far too low, or there is a mismatch such as feeding a microphone level signal into a line level input. Check the types of inputs you have and see from that if there are mismatches, you can't mix and match microphone and line level signals without converters.
If these all appear to be OK then I suspect that you are running your signals too far. This results in the loss of the high frequency signal content and is usually apparent as a muffled signal when heard. If this is the case you will probably end up having to change at least some of the equipment to go to line level signaling. Balanced signaling is ultimately the best way but cost may be prohibitive.
Data Tape erasing
Q: I manage an archive of data tapes which in some cases are not used for long periods of time. Tapes with useful information are removed from circulation and I would like to find a way of erasing the remaining tapes.
A:Most digital recording systems have a means of erasing tapes onboard. I would absolutely rely on the use of this for erasing tapes. Do not try to use degaussing devices made for video tapes – they will totally wipe all of the base format from the data tape and render it completely useless!
Source
Security Installer
