Modem Definitions

To help select a modem here are definitinss of some of the important features and controls on modems:
Attenuator - Reduces the sensitivity of a modem in order to reduce images and other effects of strong nearby signals. The sensitivity is reduced by attentuating (adding resistance) to the input signal.
Audio squelch - Suppresses the audio (or video) output of a receiver in the absence of a sufficiently strong desired input signal to prevent amplifying noise. Scanning resumes after for a few seconds.
Autoload Automatically stores new frequencies found during a search into the scanner's memories.
Bank Method of dividing a scanner's channels into smaller, manageable blocks (banks) for specific purposes and applications.
Delay Determines how long a scanner pauses on a channel for another transmission before resuming scanning.
Hold Holds up the scanning on a channel so you can monitor that channel continuously.
Search The scanner tunes through a range and stops when an active frequency is found. Useful for finding new stations and users not listed in frequency directories.
Lockout Enables the scanner to skip over a channel (lock it out) during its scanning sequence.
Selectivity is the ability to reject interfering signals. A high degree of selectivity is desirable in ubran areas because there are of the large number of radio signals there. If you live in a rural area with few stations, then greater sensitivity (the ability to detect weak radio signals) will be more important.
Selectivity is measured in kHz for a certain level of interference rejection. This rejection is measured in decibels (dB), usually at 50 dB. A "50 dB" rejection means an interfering signal is reduced to a level 100,000 times weaker than its actual strength. If a scanner has a selectivity specification of "40 kHz at 50 dB," this means signals 40 kHz or more away from the signal you want to hear are reduced in strength 100,000 times. If you live in a rural area, good sensitivity is more important than good selectivity. With fewer stations to hear, you need to be able to catch weak signals and don't have to worry as much about interference. In an urban area, the opposite is true; your main concern is in rejecting interference from stations on adjacent channels, not catching weak signals. In a rural area, narrow band FM selectivity of 40 kHz at 50 dB will usually be adequate, while in an urban environment you will usually need selectivity of 30 kHz at 50 dB or better. Signals can also "mix" in a scanner's internal circuits, producing false signals known as images. Images are an unavoidable by-product of a scanner's circuitry, but the better scanners can reject most of these phantom signals and reduce their strength. Image rejection is how this is measured, and a good scanner should have image rejection of 50 dB or greater.
Sensitivity is measured in microvolts, abbreviated mV. The lower the number of microvolts, the weaker the signal that the scanner can detect and produce intelligible audio from.
Priority channel When a signal is present on a priority channel, the scanner switches to it regardless of whether signals are present on other channels being scanned It takes priority.

See also Scanner Definitions.