As you will be able to see on the 'terms' page MASC stands for 'Marconi Advanced SCrambler.'  I hope that the information on this page will answer your questions but if it doesn't feel free to e-mail for more information.  Regards.

MASC consists of a small circuit board which can be fitted in new hand held radios when they are manufactured (such as the Philips PRP74) or can be fitted into existing police radios such as the Philips PFX, Motorola HT600 or Kenwood TK340. 

Band Scrambling is the type of encryption used by MASC  When an officer speaks a section of his/her voice is split into bands of frequencies (e.g. 100-200Hz, 400Hz - 500 hz and so on ... ) it then rearranges the order of these bands.  So that any frequencies in the range 800 Hz - 900 Hz are changed to 1400 Hz - 1500 Hz, the frequencies which were originally in the 1400 Hz - 1500 Hz band are put in the 1100 Hz - 1200 Hz band etc.  Even after this, ten times a second the order in which the bands are swopped around is changed!  The order in which the bands are changed is dependant upon what is known as the 'encryption key' a number between 1 and 274,877,906,944.  Even if you have a MASC radio it will not be able to unscramble what you listen to unless you know this key number !

When listened to on a scanner MASC sounds rather like a Drunk Donald Duck !  Sometimes the odd word can picked out like 'recieved' and officers callsings but most is indecipherable.  MASC also transmits short data bursts each time an officer presses their radio 'push to talk button' and eachtime they let go.  This data burst gives the radio's identification number which can be decoded by a computer in the police control room which is then able to know the identity of the officer calling even before the officer speaks!  This feature is popular with officers as it means if they were attacked they don't have to waste vital seconds giving their callsign because they know this is already known in the control room.

The data bursts that the radio transmits have other uses as well.  Because criminals can no longer listen in on a scanner it is possible they could attack an officer and steal his radio.  If this does happen the control room can transmit a data burst which instructs the stolen officer's radio to delete it's encryption key and shut down, rendering the radio useless !

To stop some clever criminals from transmitting a spoof data burst on a police frequency and stunning an officer just before a robbery each radio will only stun itself if the base station also transmits the radios authentication code as well.  The authentication code is also a number between 1 and 274,877,906,044 so the chances of a criminal guessing it are rather slim.  Radios with MASC can also transmit in clear voice so that they can communicate with radios without MASC or ones which have a different encryption key.  The data bursts are still heard in 'clear' to.

The Home Office and The ACOP (Association Of Chief Police Officers) recommend MASC and is now used by nearly all of the British Police Forces.  Just how much MASC is used varies, most forces only quip special squads (usually the drug squards) with MASC radios for surveillance operations.  However a few forces like Northumbria and Nottinghamshire have equipped every handheld UHF radio with MASC and now all coppers on the beat use it.  (I just think they are paranoid, personally.)  Marconi are also selling MASC to other countries at the moment. 

There is no simple way to decode (descramble) MASC  There are rumours going round, mostly on the internet, from people who have a friend, who has a friend which has decoding equipment (the chip), but personally i think they it is very unlikely.  Even if you did get hold of a MASC radio or module there is still the problem of the 'encryption key' !! 

As many will know the encryption method that MASC uses can be very unreliable in certain weather conditions.  Mostly conditions where high pressure is dominant.  This can cause distant signals to be bent back down to earth which causes interference.  When this happens it is safer for the police to use the radios in safe mode.  This causes interference, particularly towards the higher bands where the same frequency may used at two locations which are normally out of range of each other.