AM broadcast bandwidth?

[AdrianH]

Just a quick question, but is there meant to be a standard in AM broadcast transmission.

I have seen bandwidths of just over +/- 5kHz to +/- 9Khz, I am sure I could see a China radio once well above that?

I guess it depends on the audio cut off point? measurements are visual from some of the SDR's around the globe.

Adrian

[Nuvistor]

I have not read it all but this is from Ofcom quoting a maximum channel bandwidth of 9Khz, I.E. +-4.5khz, this is for LW and MW, for SW it states 10khz bandwidth, I.E +- 5 KHz.

https://www.ofcom.org.uk/__data/asse...54/IR_2022.pdf

From memory the UK is in Region 1, other regions may differ.

[Nuvistor]

Further to post #2, Region 2 includes the USA and this is from their code of Federal regulations. The channel width is 10Khz for MW, slightly broader than Region 1 but still only +- 5Khz.

[Radio Wrangler]

Region 1 has AM broadcast channels at 9kHz spacing. So the max audio bandwidth at the threshold of encroaching into someone else's channel is 4.5 kHz. In reality it needs to be a bit less to allow for filter roll-off and a rather savage filter. So, back before the channel ressignments in the seventies, medium wave did sound a lot better.

Region 2, the USA etc got 10kHz assigned channel spacing, and so, ooh! 500Hz more audio.

Note that these are channel spacings. Occupied bandwidths need to be narrower to avoid splatter into adjacent channels.

David

[AdrianH]

Thanks for the replies.

Well perhaps it is something to do with the SDR's but the visual bandwidth certainly seems to be wider for some UK MW stations then others for example:-




Thanks for the link to the document that helps.

Do they use audio limiting to 4 or 4.5kHz then to stop the sidebands or is there something else added, it is not a lot to get music through?

Adrian

[Nuvistor]

Perhaps the splatter is produced in the SDR as you say. On the other hand I wonder how much monitoring Ofcom does and relies on complaints before checking TX parameters.

The roll off although steep cannot completely be zero so unless roll off start much lower in frequency there will be some higher products. I presume these must be kept below a threshold.

[Jon_G4MDC]

It jooks like 1557 has got the Optimod cranked up a bit to compete in the loudness wars.

These were probably not playing similar program content.

[Bazz4CQJ]

With commercials stations, I wonder how the bandwidth might alter during advertisements as compared with general output? Maybe its just the levels that change?

B

[peter_sol]

There is a steep filter to limit the audio output above around 4KHZ. This is before the compressor.

[Cruisin Marine]

With commercials stations, I wonder how the bandwidth might alter during advertisements as compared with general output? Maybe its just the levels that change?

On AM it shouldn't be any different, the audio processing will cover all programme content. If they drive the processors harder, it will just introduce distortion.

The worst culprit is Television, the ad's are sometimes a good 6-10 dB higher than the programme content to still try and get the viewers attention when they go to grab a beer or make some tea.

AM audio processing is quite an art, and when set up properly is a wonderful experience. Sadly, even national stations in the UK haven't a clue nowadays in the main.

[m0cemdave]

Some HF broadcast stations have very wide audio bandwidth.

As I write this, a quick look via the Twente SDR provides the following examples:

15410 kHz, with 9.5 kHz audio (19kHz occupied bandwidth)
9600 kHz, 8kHz audio (16kHz occupied bandwidth)
9585 kHz, 8kHz audio (16kHz occupied bandwidth)

I think they are all China Radio outlets. They all sound very good when playing music.

[Cruisin Marine]

I think they are all China Radio outlets. They all sound very good when playing music.

It used to be called the wild West, now the wild East.

[AdrianH]

Originally Posted by peter_sol

There is a steep filter to limit the audio output above around 4KHZ. This is before the compressor.

I actually thought the compressor would be first as iI had assumed it may provide it's own distortion and harmonics, I live and learn.

Originally Posted by m0cemdave

Some HF broadcast stations have very wide audio bandwidth.

As I write this, a quick look via the Twente SDR provides the following examples:

15410 kHz, with 9.5 kHz audio (19kHz occupied bandwidth)
9600 kHz, 8kHz audio (16kHz occupied bandwidth)
9585 kHz, 8kHz audio (16kHz occupied bandwidth)

I think they are all China Radio outlets. They all sound very good when playing music.

Yes I was sure that China radio was a wide station as mentioned in first post, hence the general question, I have also just been checking a SDR in Orlando Florida and found the same at +/-9.5kHz on MW broadcast.



It makes the Region 1 system seem narrow.

Adrian

[Martin_Calva]

Here is my recollection from years ago... (so no guarantee that the information is correct although I think it is correct)

The AM channel spacing in Europe is 9kHz. The BBC broadcast AM with audio going up to 9kHz, so strctly speaking a total bandwidth of 18kHz. However there was generally no problem of interference between channels as the level of power at the extremes of the sidebands is very small in normal music signals. And frequencies were chosen so that stations in the same area did not use adjacent channels.

My dad told me that in the 1920's there had been discussion about this subject. P P Eckersley, the chief engineer of the BBC , had made a sarcastic comment that there would only be problems if the music consisted of a choir of bats accompanied by an orchestra of dog whistles.

The channel spacing in China is 8kHz. In the 1980's a Chinese student bought a digitally tuned AM receiver in Birmingham. I advised him to confirm before he bought it that it would be suitable for use in China. The salesman assured him it would be. Then he found that its channel spacing was not be compatible with Chinese channels. I advised him to take it back for a refund, as it was not suitable for the purpose for which it has been bought and which had been stated at the time of purchase, so under the Sale of Goods Act, the shop was obliged to accept the return.

A quick look at the Quad AM3 handbook shows that it provides control of the bandwidth of the IF amplifier. The manual states that, in the wideband position of the control, frequencies up to 10 kHz can be received.

[kalee20]

Until transmissions ceased in 1985, the sound quality of VHF television (which was of course AM) was very good. Some radios with SW could tune up to Band 1 frequencies... Im guessing that the audio extended to 10kHz-plus, so the bandwidth would be well over 20kHz?

[G.Castle]

Verify my understanding:

European channels spacing was 10kHz until sometime in the 1970's: is this wrong?

A full AM B/W of 9kHz, (both sidebands present) gives audio of 4.5kHz.

May just be having a brain fart!



Edit: someone may have to draw me a diagram with coloured crayons.
Greg.

[Synchrodyne]

This topic has come up previously, for example see:

https://www.vintage-radio.net/forum/...d.php?t=160729

https://www.vintage-radio.net/forum/...ad.php?t=70496

https://www.vintage-radio.net/forum/...=1#post1072168

In brief, historically a 10 kHz audio bandwidth (±10 kHz about the carrier) was the usual design basis for AM systems from studio through landline to transmitter. Transmitters were often specified accordingly, although some could work with wider bandwidths. Landlines were sometimes the limiting factor. Particularly at LF, transmitting aerial Q could be limiting, as I think was and maybe is the case with Droitwich. But it did not have to be if some effort were made. For example, the former Allouis radiator was designed to have a ±10 kHz bandwidth. Audio bandwidths beyond 10 kHz were also found in some cases. E.g. BBC Brookman’s Park was 15 kHz from the late 1940s.

As an aside, the 15 kHz number arrived with FM in the USA, after some initial debate as to whether the audio bandwidth should be 10 or 15 kHz.

Deliberately and severely restricted MF audio bandwidths seems to have been a European idea developed in the late 1960s, spreading to the UK in the early/mid-1970s. The original BBC limit was 5 kHz (using the FL4-55 filter), but I think it was later changed. Available papers on the topic indicate that the benefits of bandwidth restriction were not of major proportions.

In the USA, the NRSC curve in the 1990s limited audio bandwidth to 10 kHz, which did upset some broadcasters who had previously been at 15 kHz.

HF transmitters were probably specified to 10 kHz, but the “working number” seems to have been 6 kHz. In part this was determined by HF links. At least in the post-WWII period, point-to-point ISB/SSB receivers used for such links typically had 3.5 and 6.0 kHz sideband filters, of which the 6 kHz was often referred to as a “broadcast” or “music” quality filter.

Overall, there has never been a “hard” connection between AM audio bandwidth and transmitter channel spacing. Outside of Europe, 10 kHz audio bandwidths are probably still quite common.


Cheers,

[Synchrodyne]

Originally Posted by G.Castle

Verify my understanding:

European channels spacing was 10kHz until sometime in the 1970's: is this wrong?

A full AM B/W of 9kHz, (both sidebands present) gives audio of 4.5kHz.

The 9 kHz spacing in Europe goes back to the 1930s, as far as I know.

Certainly, the ITU 1948 Copenhagen Plan was based upon 9 kHz spacing, except at the very upper end, where the spacing was 8 kHz above1538 kHz, i.e. for 1546 through 1602 kHz.

In 1978, 9 kHz spacing was adopted throughout, with MF channel frequencies moved slightly to be multiples of 9, e.g. 908 to 909 kHz,

If the AM transmitted bandwidth were limited to 9 kHz, then the resultant audio bandwidth would be 4.5 kHz. But that is really academic, given that transmitted bandwidths were seldom determined by the channel spacings. Mostly there was some sideband overlap between adjacent channels, not a problem with appropriate frequency allocation planning. For example, at one time, US clear channel AM stations were protected from adjacent channel as well as co-channel interference in their primary (ground wave) service areas, thus allowing the use of wideband receivers. But their night-time skywave extended service areas were protected against co-channel interference only.


Cheers,

[Synchrodyne]

Originally Posted by kalee20

Until transmissions ceased in 1985, the sound quality of VHF television (which was of course AM) was very good. Some radios with SW could tune up to Band 1 frequencies... Im guessing that the audio extended to 10kHz-plus, so the bandwidth would be well over 20kHz?

As far as I know – needs to be confirmed - the original UK target for TV sound was a 10 kHz audio bandwidth, and that applied to the London to Bringham link. I imagine that generally, landline and link quality would have been the determining factor, but there were probably cases of above 10 kHz, maybe to 15 kHz. One might have expected the latter for the link to the Crystal Palace transmitter,


Cheers,

[G.Castle]

Thanks! Interesting information.

Regards,
Greg.