Audio for Surveillance Systems TutorialBy John Honovich, Published Oct 05, 2011, 12:00am EDT
One topic on which we regularly receive questions is the use of audio in surveillance systems, and what products and installation standards should be used. In this update, we'll cover the basics of using audio with IP surveillance systems including:
- How to Select a Microphone
- How to Select a Speaker
- The Pros and Cons of Built in Speakers
- VMS Integration
[UPDATE: See our newer, in-depth Audio Surveillance Guide]
Before connecting any audio source to a camera, first it is key to understand what types of inputs the camera can accept, and what these terms mean. Typically, audio inputs on cameras will accept either mic or line level signal. This, simply put, refers to the amount of voltage which the source device outputs. A microphone creates a very small amount of voltage when someone speaks into it, typically in the range of ten-thousandths of a volt. A line level signal, on the other hand, is approximately one volt. So, plugging a mic into a line level input will result in extremely faint audio, if any, and plugging a line level source into a mic input will result in loud, distorted audio. Some cameras may accept either, but require it to be manually switched to the proper level, either via dip switches or the web interface. Input and output levels can normally be found on the camera's spec sheet, as is shown on this spec sheet [link no longer available] for the Panasonic WV-SF346 (page two, right column). Typically, line level sources are preferred, as microphones require more gain to amplify, and introduce more noise into the signal.
There are several types of outputs in the audio world, as well. For our purposes, we typically only need to know two: line level (covered above) and low-impedence (4- or 8-ohm). Most cameras will output line level only, which must then be connected to a powered speaker. That is, a speaker with a built-in amplifier, which must be powered separately. Some new products, such as the Axis P8221 I/O module also have low-impedence outputs, in Axis' case, a 4-ohm speaker output. This allows a speaker to be directly connected to the I/O module, with no external amplifier or power required, which makes cabling much simpler.
When selecting a microphone, we must consider how the microphone will be used and where it will be mounted, which determines what pickup pattern is required. Microphones may be either directional or omnidirectional, depending on what is required. If mounting a microphone in the ceiling of an interview room, for example, we most likely would want to use an omnidirectional microphone. If mounting a microphone in a wallplate near an entry door, however, we could use a directional, hemispherical microphone, since it does not need to pick up audio in all directions.
In selecting speakers, again, first consider what the speaker will be used for. Which will determine its size and power. If it's simply to talk to a visitor at the door, a few watts from a 3" or 4" speaker will be more than sufficent. If the speaker is to be used to alert subjects in a car dealership lot afterhours that they're under surveillance, a 15-30 watt horn is more appropriate, for better throw distance and volume. Manufacturers such as Bogen and Valcom are the largest distributors of powered speakers. Sony also recently released their own powered horn for use with surveillance systems, which accepts 24VAC power and line level input. This is useful since most powered systems require DC power supplies, while 24VAC is more likely to be used for surveillance.
Many manufacturers, such as Panasonic [link no longer available], ACTi, and Vivotek provide models with biult-in microphones. Cameras such as the Axis M1054 and Brickcom CB-100A [link no longer available] cube cameras take it one step further, building a speaker into the camera. Built-in audio is, undoubtedly, the simplest way to add audio to the surveillance system, as no external connections are required. However, it lacks the same flexibility and choice of equipment of other cameras. Installing a camera with built-in audio may be a good choice for a secure vestibule, since it's a small, enclosed area. Relying on these microphones in an interview setting, however, may not provide the quality of audio needed or desired. Built-in audio is not really a common feature, with very few of the big names providing cameras with this functionality.
Most of the major VMS players support bi-directional audio from IP cameras. However, this support may vary from manufacturer to manufacturer, and from model to model within a given manufacturer's line. Always verify the needed audio support is there before selecting cameras. Looking at Genetec's compatibility list, for example, we can see that some models don't support audio, some support audio in, and some support full bi-directional audio.
Some systems also allow a microphone or speaker attached to one camera to be associated with other cameras. This may be handy for an interview room equipped with multiple cameras, for example, allowing the full interview to be seen/heard on each of them. Associating a speaker with multiple cameras would allow an operator to alert intruders in a perimeter, for example, no matter which of the perimeter cameras he or she was viewing at the moment.
Normally, the VMS client supports this audio by adding a push-to-talk button associated with the camera. Pushing the button activates the speaker associated with the camera, and the operator is able to speak through a PC-connected microphone. Depending on setup, the microphone attached to the camera may be left open at all times, or triggered on event. Again, this varies based on the client software, and features may vary from manufacturer to manufacturer.
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