Easy to Understand Technical Facts About the AM Band on Your Radio

(Re-post)

How good, or bad, is the AM band, especially in the context of my complaint that the Sangean K-200 sounds muffled on AM? It all depends. Here are some solid technical facts from our readers:

Gary writes:

Excessively muffled sound on AM is mainly due to:
1. An IF filter that is too narrow.
2. A poorly designed audio stage, a poor speaker, or inappropriate equalization. Full range bass and treble controls can often help.

It's true that FM has much wider bandwidth than AM, but it's also true that not all radios sound muffled on AM.

Ed adds:

AM radio has more than enough quality for voice. A telephone has less than a 3 kHz bandwidth, and yet you can still instantly recognize the voice of any of your friends or family. The trouble is when little speakers are bass-boosted ("voiced") in order to sound a little warmer on music, it winds up making voices sound boomy/muffled.

Demetri adds:

AM's source material's upper end is hard limited at 10khz. With single filter radios, the filter width must balance good adjacent channel rejection with fidelity so some fidelity is sacrificed in most designs. Also, the radios that sound good probably also artificially raise the high end to give that 10khz every chance to sound as good as possible.

FM source material is limited to 15khz for each channel so you get quite a bit more on the high end. Also the channel spacing is much wider on FM so the filters will not limit the fidelity as much.

Remember FM as 108-88=20Mhz to play with but AM only has 1.710-.530=1.2 MHz to fit all those stations.

An example of a high performance AM radio from Brian AKA Scooby:

My SSTran AM transmitter broadcasts a rather wide bandwidth signal. When coupled with a good tube radio, such as my 1956 Blaupunkt set, the sound is astonishingly good. The Blaupunkt in question has both AM and FM. I won't deny that the FM still has a very slight edge in fidelity, but the average listener that hears it thinks that the radio is playing FM instead of AM from my SSTran.

The flip side to this is that commercial stations aren't allowed to broadcast using the bandwidth my SSTran uses. Not to mention that the AM is still prone to interference, no matter how wide the broadcast bandwidth or quality of the AM receiver.

Mark Roberts adds:

Don't forget about the sharpness of the bandwidth filtering. Vintage radios, pre-ceramic filters, had very gentle roll-off, giving them a pleasing mellow sound. Modern ceramic filters are sharper and can have side effects beyond restricted frequency response. My AM HD tuner has an extremely sharp cutoff at 5 kHz, the proverbial brick wall. There are all sorts of knock-on effects and distortion that result. It has a little better fidelity than the typical AM radio but it sounds awful, sort of like the way network radio broadcasts used to sound -- even if they were on an equalized line -- before they went to satellite delivery.

Comments

[GregS]

AM's narrow 10 kHz bandwidth is a problem, but it is compounded by the lousy speakers and cheap electronics that go into most consumer-level AM radios, and the poor selectivity and sensitivity of those radios. AM's sound quality is actually not bad when you hear it on a well-made receiver with a good quality speaker.

[Doug T.]

There is a serious flaw in the above description.

In the US, broadcast band AM is assigned at 10kHz chunks, but that is a full AM signal -- both upper and lower sideband and the carrier. That means that the upper audio limit is 5kHz.

Here is a web page that sets out the physics of this in a fairly straightforward way:

http://www.radio-electronics.com/info/rf-technology-design/am-amplitude-modulation/spectrum-bandwidth.php

Here is another:

http://web.physics.ucsb.edu/~lecturedemonstrations/Composer/Pages/76.24.html

This may not be a problem for speech (almost all human speech can be captured under 4kHz) but it is most definitely a problem with music.

[Mark Roberts]

Actually, the current audio bandwidth IS 10 kHz. That's the NRSC standard...look it up. You (Doug T.) are confusing transmitting bandwidth with allocation. AM stations in the same vicinity are allocated no closer than 30 kHz.