Today, the analogue broadcasting of television signals has become obsolete. Proof: the RTBF has moved on to Digital Terrestrial Television (DTT or DVB-T) since 1 March. However, this transition to digital technology is not without consequences for users of wireless microphones which still need analogue broadcasting.
We take a look at this Hertzian problem with Gaëtan Grenier, managing director of WNM, a Liégeois company specialised in audio.

What difference is there between digital or analogue Hertzian broadcasting for television?

Gaëtan Grenier: Broadcasting for Hertzian television is divided into emission channels. The television channels are situated between 470 and 860 Megahertz (MHz). Each channel has an 8 MHz bandwidth and is numbered from 21 to 69. Digital television signals are broadcast in the same areas on the spectrum (frequency) as their analogue predecessors. Transmissions via a wireless audio connection also occur within this range on the spectrum. Users of these wireless connections can use the analogue television channels, as secondary users. Analogue television only uses 7 out of the 8 MHz on a UHF channel in Europe and in Africa. Until now the 1 MHz broad "gap" was used for communications, report transmitters and, to a certain extent, also for wireless microphones.

analogue signal corresponding to a TV channel

Accordingly, the UHF frequency range was divided between analogue television broadcasters and wireless audio transmission devices, before the introduction of digital television.

Previous utilisation of the UHF frequency range:
division between analogue television chains and wireless audio devices.

Analogue TV channel (8 MHz width)
Transmission frequency of a wireless microphone (200 kHz width)

However, digital television takes up the entire 9 Mhz of an UHF television channel. The new digital signal's spectrum used by DTT no longer “leaves” that 1 MHz gap. On an 8 MHz channel, when using digital, it is possible to multiplex up to 8 television channels in SD format.

Television channel with DTT signal

Especially because of the simultaneous use of analogue and digital television transmitters, the spectrum available today for wireless microphones is seriously restricted in numerous countries.

Current exploitation of the UHF range by the analogue
and digital television channels as well as by wireless audio transmission devices.

Analogue television channel (8 MHz width)
Frequency emission for a wireless microphone (200 kHz width)

Digital television channel (8 MHz width)

Moreover, there is a growing international tendency to restrict the part of the UHF spectrum reserved for television channels, in favour of new services such as DVB-H (Digital Video Broadcasting – Hand-held, mobile digital video). New rules which make it easier for new priority players ( for example mobile telecommunication service providers) to regularly obtain unused frequencies will even further reduce the number of frequencies which are technically exploitable by wireless audio connections.

Future use of the UHF range by digital television channels,
by wireless audio connections and by new services.

However, the number of wireless audio connections multiplied the last few years, for the production of spectacles as much as for television and sports shows. This does not only concern the wireless microphones we most often associate with this but also for wireless listening systems and wireless interphony systems.

Why did this transition to digital happen?
G.G.: First of all, because the image quality is clearly better. Moreover, a European directive imposes the suppression of all analogue Hertzian television broadcasting by 1 January 2012. In Belgium, given how the entire territory is cabled, most of the viewers receive their images via cable distribution (analogue or digital with a decoder – VOO, TELENET) or via their ADSL line (Belgacom TV and Numéricable). Moving on to digital only concerns a tiny part of the population but it is a public service obligation of the RTBF and VRT to be broadcast via Hertzian channels. RTL for example has no obligation whatsoever to make programmes in DTT since it is exclusively broadcast by the cable operators.
This means that in Belgium there are currently only two multiplexes: one for the RTBF (La Une, La Deux, La Trois and Euronews) and one for the VRT (Eén, Canvas/Ketnet, Canvas+/Ketnet+). The problem was different in France. Cable was and is still sparsely present outside of the large agglomerations. Even more, analogue broadcasting didn't cover the entire French territory. DTT partially solved this problem. Today there are six multiplex packages. For the zones which would not be covered by the DTT transmissions, a “DTT per satellite” was even set up, which broadcasts the equivalent of those 6 packages per satellite. Thanks to this technology new channels emerged which are now seriously competing with the “big” historical channels (TF1 notably).

Still, the number of channels is rather high considering the number of multiplexes today... (69-21=48 channels)
G.G: yes and no. Each TV transmitter covers a geographical zone related to its strength and the topography of the area (notably its relief). To ensure that the entire territory is covered, multiple transmitters must be set up to relay the same TV programme (be it analogue or digital). But, every close transmitter cannot be on the same TV channels as its neighbour. Example: for the only DTT multiplex in Wallonia, the RTBF alone needs no less than six TV channels (45, 56, 61, 64, 65 and 66) to make sure the entire territory is covered! If you add to that the channels used by the VRT multiplex as well as the channels used at the borders by our neighbours (waves do not respect any borders!), we do not have much breathing space for our wireless equipments.

Why is this a problem?

We still use analogue transmitters. However, these require large bandwidths. We need around 200 kilohertz (kHz) for a wireless microphone, for example. This can seem little but you have to multiply those 200 kHz by the number of microphones and other wireless systems which are used during a show or on a television set. It is not exceptional that over twenty frequencies are used at the same time, and that number can rise to over hundred for big international events, for example when several television channels are present (big sports meetings, track events, bicycle races, large concerts, etc.)
Actually, to develop the Belgian example, the IBPT (Institut Belge des Postes et Telecommunications – Belgian Postal and Telecommunications Institute) , the organisation in charge of regulating this field, allocates us three television channels for our activities: the channels 27, 29 and 69. That means 3 x 8 MHz or 24 MHz for everything. These channels are so-called 'free', this means everyone can use frequencies without prior authorisation. From time to time it is still possible to obtain a special dispensation for other frequencies but this becomes rare. And even for the 'free' channels we see the start of restrictions: channel 29 can no longer be used in the Hainaut and channel 69 is no longer allowed in West Flanders!
Moreover, the 'free' frequencies dedicated to our equipment are anything but standardised on the European level. Every country allocates its TV channels in a different way. Our equipment is technically incapable of functioning on the whole of the UHF range (or, as a reminder, from 470 to 870 MHz), it usually has an operating “window” which ranges from 24 to 36 MHz or the equivalent of 4 or 5 contiguous TV channels. If, while we are on the road, both in Belgium as in Europe in general, those 4 to 5 contiguous channels are used by TV broadcasters, then our equipment stops functioning.
To guarantee that we are capable of making our machines work on the entire available range, we would have to multiply our investments in material by fifteen to cover the entire spectrum; commercially this is obviously unthinkable.
Thankfully the manufacturers are beginning to work on solving this problem by offering larger coverage “windows” (we are talking 180 MHz) which would mean more flexibility and flexibility of use also for the service providers.
Unfortunately, like we said at the beginning, the mobile phone operators, for example, are eager to recuperate the last available TV channels to offer their subscribers new services, which would restrict or even remove our access to the UHF band. These operators obviously weigh in (notably financially) on the decisions of national and European lawmakers. Even if the equipment manufacturers and service providers manage to rally together, it is uncertain whether they can really make themselves be heard.

Could the wireless audio broadcasts not be done in digital?

G.G.: Currently, there is still a latency due to the encoding/decoding. However, audio does not permit any delays. Sound transmission must happen live. Imagine a concert where the singer's microphone would transmit the sound half a second late. The singer would hear himself delayed which would make it impossible to sing. The same goes for a journalist who has to hear himself while interviewing live during a news report.
Some manufacturers are working on the problem. But the results are still in their early days. We do not expect any significant advancement in this domain before at least two years.

This seems rather paradoxical. Would broadcasting be detrimental to the production?

G.G.: Absolutely. Great-Britain is one of the most advanced countries in the world when it comes to digital broadcasting. The broadcasters had to react for the 2012 Olympic Games in London: there wouldn't have been any channels left to receive the television channels from all over the world (around 1300). The authorities decided to free up certain channels to allow the televisions to use their wireless connections!

What are some possible leads for a solution?

G.G.: The lawmakers urge us to go higher up in frequency (above 1800 MHz). But with analogue, once you reach those frequencies the waves become so small that they no longer go through obstacles and only reach very short distances.
In digital that could work, but at this time the latency due to the encoding is still too much of a problem.

Sources : Sennheiser