AM STEREO IN EUROPE (RWI OCTOBER 1991).
This was the first article I wrote for Radio World International after meeting some of the editorial staff at a broadcast radio exhibition in Central London in July 1991. The contents of the article looks incredibly out of date today, but back in the early 1990s digital radio was still very much embryonic and attempts to improve the performance of AM were quite a priority…. The article is reproduced below courtesy of Radio World International and is as published, except that I cannot resist the temptation to put it back into something like the original British English, with ‘s’ spellings instead of ‘z’, plus not starting sentences with conjunctions such as “but”!
Author’s Article Title: AM Stereo in Europe
[Originally Published in Radio World International
(Vol. 15, No. 20, p.p. 1&11) 23rd October 1991.]
Text as published under the heading:
Debate On AM Stereo In Europe – AM Stereo On Agenda
Lawrie Hallett.
WORCESTERSHIRE, England. AM stereo broadcasting is an accepted practice in many parts of the world, but European broadcasters showed little or no interest until recently.
Since 1989 commercial broadcasters, notably in the U.K. and the Republic of Ireland, have started examining AM as a way of improving the “perceived quality” of medium wave.
[However] Germany and France appear more reserved, preferring to keep their options open for the introduction of more data services or the launch of an as yet undefined AM Radio Data System (RDS) standard.
Commercial broadcasters, including some in France, however, would prefer to have AM stereo now, because it is a proven system that has obvious marketing advantages. Conversely, AM RDS is not developed and could be 10 years or more away from commercial introduction.
Looking Toward Receivers
The de-facto AM stereo transmission standard worldwide is [Motorola’s ] C-QUAM. [However] European broadcasters generally believe that receiver availability is of paramount importance when launching AM stereo.
Japan, the major receiver producer for the European market, has just accepted C-QUAM as its domestic standard and appears poised to support its introduction in Europe, when and if this occurs.
C-QUAM AM includes a stereo component and the traditional monophonic AM component that consists of the carrier frequency and amplitude modulated side-bands.
A common technique for getting more information out of a fixed bandwidth is to implement a second carrier on the same frequency but 90 degrees out of phase, or in quadrature with the original. C-QUAM utilises the the second carrier to convey the stereo information.
There is a potential compatibility problem with AM stereo because there are other uses for a quadrature carrier such as data or AM RDS. Some long-wave transmissions already carry data for remote industrial control and switching.
In order to make C-QUAM compatible with existing AM radios, the left and right channel information is matrixed. The left-plus-right information amplitude modulates the normal AM carrier and the left-minus-right information modulates the quadrature carrier.
An Evelope Detector
If most existing AM radios did not use an “envelope detector” that picks up the additional information as distortion, simple matrixing of the two carriers would provide excellent AM stereo.
Introducing stereo in quadrature adds very little extra noise (unlike FM stereo) and occupies essentially the same bandwidth. But backwards compatibility is the key to future acceptance of a new standard. AM stereo has to be compatible with existing mono receivers.
The Motorola [C-QUAM] system adjusts the envelope amplitude to ensure that stereo transmission is not seen by envelope detectors as distortion. This is accomplished by multiplying the signal by a constant during transmission and then reversing this operation in the receiver.
The result is a high quality AM stereo system. The system is easy to implement at the transmitter site and in receivers. It is also spectrally efficient and will not affect existing mono radios.
European broadcasters have some difficulties which do not arise in many other parts of the world.
Unlike in the U.S., medium wave band channels are 9 kHz, not 10 kHz apart. Additionally, because of spectrum crowding, modulation is severely restricted with filters rolling of frequencies just below about 5 kHz.
The filters are used to ensure that side-bands are at least 40 dB down by 9 kHz away from the carrier frequency. If they are used in conjunction with stereo transmissions the apparent separation performance is not as impressive.
However, in practice, there is no reason why sharper filters such as those used in the Orban Optimod processor cannot be used to achieve the required -40 dB at 9 kHz removed from the carrier without beginning to roll off frequencies until 6 kHz.
This makes all the difference and stereo performance becomes not unlike that of stereo FM. Obviously, high frequency response is not as good, but separation levels of between 35 dB and 40 dB are easily achieved.
Receiver Design
In fact, AM stereo in some cases can be more satisfactory than FM. In particular, car listening benefits from less dynamic range, no multipath distortion and no extra noise caused by the presence of stereo information.
It may seem exaggerated that the extra approximately 1 kHz of sideband information can cause such improvements. In fact, the measure of AM stereo performance is more critically determined in the receiver design.
Measurements on a variety of mono AM radios has shown that frequency response most often begins to roll off at about 2.5 kHz, with some more expensive or older designs as high as 4.5 kHz.
C-QUAM receivers begin by using a low distortion synchronous detector, which is vastly superior to the traditional envelope detector. The Motorola chips are in their third generation with adaptive bandwidth and variable notch filters to remove the worst effects of adjacent channel interference.
These features, coupled with a full 6 kHz frequency response,provide AM reception remarkably better than that achieved by most conventional designs.
Now that Japan appears set for full-scale C-QUAM receiver manufacture, Europe has perhaps its last chance to upgrade the quality of service provided to AM listeners.
In the U.K., commercial broadcasters are anxious to see introduction go ahead quickly since the general feeling is that with the introduction of digital audio broadcasting coming nearer, it is only a matter of time before the focus for AM changes.
Lawrie Hallett [at the time of writing, was] a partner in the engineering and consultancy firm, Phoenix Communications, which specialise[d] in radio and studio design and installation. Contact him in Worcestershire, England at telephone +44-386-xxxxx; Fax +44-386-xxxxx, or circle Reader Service 21.
[First Published on this site: 24th January 2016.]