Digital Video Broadcast Terrestrial and Wireless Microphones
Interaction
For some years now, the introduction of digital terrestrial television (DTTV), also called DVB-T (Digital Video Broadcast - Terrestrial) in Europe and DTV (Digital Television) in the United States, has been progressing rapidly throughout the world.
DVB-T is an international transmission standard for terrestrial digital television. Digital TV signals are transmitted in the same frequency range as their analog predecessors. This is also the range in which RF wireless audio transmission technology operates. Users of wireless audio transmission are able to use the analog TV channels as secondary users. In Europe and Africa, analog television occupies only 7 MHz of the 8 MHz wide UHF channel. Until now, the remaining 1 MHz gap has been used for communication, reporting transmitters and to some extent for wireless microphones.
Therefore, before the introduction of digital television, the UHF frequency range was shared by analog TV transmitters and wireless audio transmission equipment.
Digital television, however, completely occupies the 8 MHz wide channel. Due to the form of the new digital signal that DVB-T uses, the 1 MHz gap is no longer available.
Problems
Especially due to the simultaneous operation of analog and digital television transmitters, the spectrum that is now available for wireless microphones is severely limited in many countries.
Furthermore, there is a growing international tendency to restrict the range of the UHF spectrum available for TV channels in favor of new services such as DVB-H (Digital Video Broadcasting - Handheld). New regulations that facilitate the granting of non-regularly used frequencies to new primary users such as mobile communications service providers will further reduce the frequencies that can be technically used for audio transmission.
It is clear that a further reduction in frequency resources for audio transmission is to be expected.
Solutions
Television has a primary status in the VHF and UHF range; wireless microphones are merely secondary users. Therefore, the operation of wireless microphones is only permitted if it does not interfere with television reception. The operation of a wireless microphone within a digital TV channel can result in interference (for example, image "freezing" on the television screen). As a general rule, we can say that the operation of wireless microphones in the same channel is not permitted as soon as television reception by a DVB-T receiver is possible. In order to operate reliably in the same channel, wireless microphones would require a much higher signal level than the digital TV signal.
Measurements carried out by us in various cities have shown that the operation of Sennheiser wireless microphones in channels adjacent to powerful DVB-T transmitters is possible. The distance between the filter flank of the DVB-T signal and the carrier of a wireless microphone is between 400 kHz and 1 MHz and is dependent on the quality of the filter of the DVB-T transmitter.
The digital TV signal has a completely different signal structure and spectrum than that of the analog television signal. There is easily a risk of users working in an occupied digital TV channel without noticing it, as the audio signal at the microphone receiver during operation in a channel is virtually indistinguishable from the usual white noise. Therefore, noise is not an indication of a free channel.
When scanning for unused frequencies, many RF wireless receivers only indicate the levels of stable FM signals, while DVB-T signal levels may not or only partly be recognized. Our evolution wireless G2 receivers, on the other hand, allow a good indication of DVB-T levels. Another efficient way of finding digital TV signals is to use a DVB-T receiver set-top box instead of expensive spectrum analyzers.
The test operation of DVB-H (Digital Video Broadcasting - Handheld), for example in Berlin, for television transmission to cell phones and PDAs also clearly shows that fewer and fewer frequencies for the wireless transmission of audio signals will be available in the future. In order to make optimum use of the frequency ranges for microphone channels, our new SIFM (Sennheiser Intermodulation Frequency Management) software can be used to calculate intermodulation-free frequencies for multi-channel RF wireless systems.
If one wants to accommodate a large number of microphone channels in such a "full" spectrum, high selectivity and high intermodulation resistance are increasingly important in order to ensure interference-free multi-channel operation.
A high intermodulation resistance of a receiver guarantees a high transmission reliability and dense occupation of the frequency spectrum. Very effective filters that allow our receivers to "see" only one particular frequency range (i.e. group of frequencies) at a time are able to filter out the interference, thus allowing excellent selectivity.
Although greater flexibility in the choice of frequencies can be an advantage at times, it can however also reduce the resistance to interference. This is particularly the case in environments that are exposed to strong RF signals. A wide switching bandwidth opens up a large "entry window" to other unwanted signals (such as digital TV channels and other wireless microphones) as well as their intermodulation products.
Since we attach great importance to achieving the best possible operating reliability and transmission quality, our products from the 5000 and 3000 Series as well as our evolution wireless G2 microphones are able to ensure reliable operation for even the largest professional multi-channel systems for our customers throughout the world.
An audio signal that is periodically exposed to interference from a digital television signal.