I think it was Billy Connolly who said that he did not need to learn Algebra, as it was unlikely he would ever visit there. The Simple Measures Achieve Trouble-free Viewing (SMATV) article in Riggers 5 ventured to go there. The article referred to a need to avoid +and- 1, 5 and 9 channel relationships when designing a channel plan for a distribution system. This design principle is based on sound technical principles and here Tom Carnie, Product Manager, ventures further.

Modulation and Sidebands (n+/-1)
An amplitude modulation system, as used for the vision component of Analogue Television will produce sidebands above and below the carrier frequency extending out to the maximum modulating frequency and no further. The FM mono sound carrier at 6MHz above the vision carrier, just above the 5.5 MHz vision sidebands, in practice defines the spectral limit of the modulator's output. (An off-air broadcast signal will also have a Nicam digital carrier centred approx. 0.5 MHz above this).

A simple modulator as used in a Digibox or VCR produces a double-sideband modulated signal with vision sidebands and 6 MHz sound carriers symmetrically above and below the vision carrier frequency but sidebands are not produced 5 or 9 channels away as a look at a Digibox output on an analyser will confirm.

In Analogue Television transmission, to conserve bandwidth and spectrum, the lower sideband of the transmitted broadcast signals is heavily filtered out leaving only a vestige, hence vestigial sideband (vsb) modulation. Professional grade modulators for distribution systems also produce a vsb output.

Channel relationships
The requirement to avoid +&- 1 is straightforward and is due to the inability of some older TV receivers to sufficiently reject adjacent analogue channels. (This does not apply to DTT multiplexes, because of their different level and type of spectral energy, and are routinely broadcast adjacent to analogue). The problem with +5 and +9 relationships is related to the IF frequencies used in the receivers.

Local oscillator interference (n+/-5)
The front-end of a Television receiver operates on the Superheterodyne principle. The incoming signal is mixed with a local oscillator to produce constant IF frequencies on which the vision and sound components progress through the receiver.

The Vision IF frequency used is 39.5MHz and is produced by mixing the incoming signal with a local-oscillator which always runs 39.5MHz higher than the vision carrier of the tuned channel. The difference product, ‘f’ oscillator minus ‘f’ carrier, is then filtered out (usually these days by a SAW filter) to produce the vision ( as well as colour and sound) IF.

With UHF channel spacing of 8MHz, the local-oscillator of a set tuned to channel n is therefore running only 0.5MHz below channel n+5 vision. Taking a practical case with a TV set tuned to Ch 40 with vision carrier at 623.25MHz. The local oscillator will be running at 623.25 + 39.5 = 662.75MHz which is 0.5 below Ch 45 vision at 663.25MHz.

There are recognised limits on local-oscillator radiation directly from TV receivers and at their aerial sockets, but television receivers in a block of flats, for example, could well be operating back to back a few feet apart through a wall with their aerial feeders sharing a common route and their aerials sharing a common mast. This local oscillator leakage may well cause patterning on pictures.

The problem is best illustrated by again looking at a practical case. Consider a set tuned to channel 40. The local oscillator will be running at 623.25+39.5 = 662.75MHz. If any sound carrier from channel 49 reaches the mixer it will produce the product 701.25-662.75 = 38.5MHz which is only 1 MHz away from the vision IF and could cause patterning.

One of the tasks of the TV receiver front-end is image channel rejection to minimise the level of any n+9 image channel reaching the mixer. Over the years the performance of image rejection has been substantially improved and most modern receivers can tolerate quite high levels of n+9, a fact which has allowed relaxation in terms of Broadcast spectrum planning. It can be usefully demonstrated on the bench with a suitable vsb modulator, combiner and spectrum analyser set-up.

Thanks to a number of readers, in particular Bill Wright of Wright’s Aerial, who sent their comments in about last issue’s article. My grateful thanks also to BBC Resources Department of BBC Scotland and in particular Mr. Noble MacPherson, whose response forms the basis of this article.