Chassis size- Refers to the size of the entire radio. The most common sizes are DIN, DIN and a half, and double DIN. While single-DIN headunits will fit in virtually any car, some cars (many GM and Chrysler vehicles) have dashboard openings designed for a larger unit. Older cars may have "two-knob" chassis’. With the use of vehicle-specific trim kits, most radios can look perfect in your car.
CD changer controls- Many headunits have buttons that will control a CD changer of the same brand. The nice thing about this is that you can add a changer to your system invisibly—there is no way anyone can tell you have a changer trunk.
Dolby® noise reduction- By far the most common type of noise reduction is Dolby B, which can be found on virtually any store-bought prerecorded cassette. When used on a tape that has been encoded with Dolby B, tape hiss is cut in half. It is a common misconception that Dolby noise reduction removes high frequencies, when in fact it is merely returning them to their proper levels.
Frequency response- On a headunit, this usually refers to the performance of the CD or cassette rather than the radio. The closer a headunit can get to the full audible frequency range of 20Hz to 20,000Hz the better, but frequency response specifications that don’t include the tolerance (how many decibels of variation there is, expressed as +/- x dB) are less meaningful than those that do.
Loudness- This circuit is designed to compensate for the insensitivity of our ears to low and high frequencies at low volumes. Engaging this feature at low volumes will make the music sound more full. Many loudness circuits will reduce their effect as the volume increases.
Peak power- Measured in watts, this is the specification used most often when rating the power output of head units (in-dash tape and CD players). Peak power is a measure of how much power an amplifier can produce for an instant. This should never be confused with the much more meaningful and useable RMS power specification that measures the continuous power output of an amplifier.
Preamp or RCA outputs- These outputs use RCA connectors and provide an unamplified signal that is affected by preamplifier controls like volume and tone. The most common use for these is connecting an amplifier.
Preamp output voltage- This refers to the strength of the signal coming out of the preamplifier outputs of a headunit. Typical voltage output is around .75–1 watt, but some headunits can produce as much as 4 and even 8 volts. The advantage of these higher voltage outputs is that you don’t have to turn the input gains on your amplifier as high, and that translates into lower noise.
RMS power- Stands for root mean square and is used to indicate continuous or average power output from an amplifier in watts. Power specifications rated this way are much more reliable and meaningful than peak power specifications. All things being equal, the more power the cleaner the music.
Random play- When this feature is engaged, songs on a CD will play in a random order.
Signal-to-noise ratio- This is an expression of the mixture of music to noise. The less noise, the higher the number. Cassette tapes without Dolby noise reduction have a S/N ratio in the 50dB range, whereas CDs can score in the 90dB range. Higher decibel ratings are better.
Tuner Sensitivity- This is a way of quantifying a radio tuner’s ability to receive weak stations. In this case, lower numbers are better.