Why AM Stereo - surely it can't be anywhere near as good as FM Stereo? WRONG!

AM Stereo was developed in the 1970's and is a newer technology than FM Stereo (developed in the 1950's). There were many standards developed, including Kahn, Magavox, Harris, and CQUAM.

CQUAM (developed by Motorola) became the accepted standard in the end, and is used in USA, Australia, Canada, Japan, Mexico and other countries.

I have been experimenting with an AM STEREO transmitter. The design I'm using is a "Lite" version of a full spec design. "Triple 9 - Stereo AM" can be heard on 999kHz in Crofton Downs.

I got involved in AM Stereo as:

a). I was curious how it works

b). I couldn't believe you can get HiFi sound on AM

After receiving a ready made AMS transmitter from a friend in the USA, along with an AMS walkman, I am amazed that, amongst other things, the sound quality and channel seperation is noticably better than FM Stereo. I also tried to convert my Technics ST-610 AM/FMS digital tuner to AMS using a circuit board not much bigger than a 50c coin, and again was blown away by the sound quality - even mono AM stations come through in full Hifi (although mono). Note though, that it's not a good idea to solder the decoder to the power supply points in the Technics with it plugged in - as I realised too late and one dead microcontroller chip later (unit wasn't OFF - just in "Standby"). Luckily, the chip in the Technics is easy to replace.

AM stereo isn't used in NZ, although RNZ AM Transmitters are AMS ready and their stereo signal may be switched on I'm told.

There is no limitation to the fidelity of AM radio.  From a mathematical standpoint, AM does better in frequency response than FM.
- Leonard Kahn, 1982

Please visit the offical AM Stereo website for more information, and Alfredo's site for circuit ideas and how AM Stereo works.

2/6/2001
One thing about AM radio is that as you're broadcasting on a lower frequency than FM, you need a different type of Aerial system. I've been using the TV aerial on the roof so far, which isn't ideal at all.

Today, I set about putting the Transmitter inside a shielded box (in this case the remains of a dead CD Writer), and planning the aerial system which involves a garden tap outside. Below is a view of the AM Stereo Transmitter in it's new housing.

This should hopefully solve a major problem I've been having. HUM - lots of it and annoyingly LOUD.

4/6/2001
Finished the aerial system. It is made up as follows:

a). A cable going from the transmitter ground to a garden tap outside.

b). A tank circuit made up of about 400 turns of 0.25mm copper wire wound tightly around a toilet roll, with a 0.1uF ceramic capacitor in parallel.

c). The tank circuit connected to the aerial socket on the transmitter going via a cable to the ground point outside, and another cable of equal length connected to the aerial socket as the actual aerial.

Results: NO HUM!!! Plus, coverage is a good as before.

Next step: An RF amplifier to cover more area.

9/6/2001

Completed a kitset AMS/FMS radio. Kitsets are available from this guy.