DVB-T DVB-T transmissions – interference with adjacent-channel PAL services R. Poole BBC Research & Development In the UK, many of the new digital television (DVB-T) services are broadcast in adjacent channels to existing PAL services. There have been reports of PAL reception suffering as a result, possibly because the maximum DVB-T sideband levels have been incorrectly specified. This article describes how to calculate the PAL picture impairment arising from the presence of DVB-T sidebands. It also compares the calculated predictions with experimental data. The conclusion is that the sideband specification is correct: critical viewers would just notice worst-case interference. However, it is possible to misinterpret the specification. An allowance must be made for the difference in effective radiated powers between the DVB-T and PAL transmissions. An example is given of how a mistake could be made. Introduction There have been reports that some of the new digital television transmissions have been caus- ing interference to existing PAL services in adjacent channels. Such interference could result from three possible mechanisms: inadequate adjacent-channel selectivity of the PAL television receivers; overloading of the receivers caused by the additional signals; generation of spurious sidebands within the DVB-T transmitters themselves. Work has already been carried out on the adjacent-channel performance of domestic PAL receivers: BBC R&D, for example, has carried out practical tests on receivers which were reported in an internal technical note (these tests are referred to at several points in this arti- cle). If the interfering DVB-T signal is “clean” or “ideal”, interference only becomes visible when its level exceeds that of the PAL signal. Since the notional transmitted power of DVB-T is typically 20 dB below that of PAL, receiver selectivity would seem adequate to avoid wide- spread problems. The performance to be achieved by “real” DVB-T transmitters is specified in Chapter 11 of the “DBook” [1]. The appropriate spectrum mask, shown in Figure 11-4 of that document, was “borrowed” directly from the original ETSI specification of the DVB-T system [2]; no work was carried out within the DTG — the authors of the “D Book” — to verify the mask. The question Original language: English English Original language: 24/6/99. Manuscript received: EBU Technical Review - Autumn 1999 1 R. Poole DVB-T now arises as to whether the mask adequately protects adjacent-channel PAL services. If the mask is “wrong”, DVB-T adjacent-channel emissions could cause interference, even though the transmitters are within specification. To answer the question, a spreadsheet has been developed which allows the transmitter spec- trum mask and the receiver selectivity characteristics to be entered. From this information, the spreadsheet calculates the video signal-to-noise ratio at the output of the receiver, and hence gives a measure of the subjective picture quality. The relationship between signal-to- noise and picture quality is based on a CCIR recommendation [3], as this is the generally accepted standard. However, further work has been carried out within BBC R&D — see [4] for instance — and mention of this will be made where appropriate. Although the spreadsheet was designed for a specific purpose, it could be used more gener- ally for analyzing adjacent-channel performance. The spreadsheet is therefore described in some detail. The approach This article starts with a description of the spreadsheet design. Although reference is made to “the spreadsheet”, two spreadsheets were actually developed: one for the case where the DVB-T transmission is in the upper adjacent channel to the PAL, the other for the case where it is in the lower adjacent channel. The two spreadsheets are very similar, but not identical. It is assumed that the PAL and DVB-T transmitters are radiating with the same effective power 1, because the basic spectrum mask given in [1] makes the same assumption. However, the mask also specifies a scaling factor to be included where the powers are different. For example, if the PAL signal is really 20 dB greater than the DVB-T signal, the DVB-T adjacent- channel sidebands are also allowed to be 20 dB greater 2. After giving a description of the spreadsheet design, the article puts the spreadsheet to use by establishing the subjective picture quality for the following cases: DVB-T in the upper adjacent channel, receiver with “ideal” filtering; DVB-T in the upper adjacent channel, receiver with “real” filtering; DVB-T in the lower adjacent channel, receiver with “ideal” filtering; DVB-T in the lower adjacent channel, receiver with “real” filtering. In all cases, the DVB-T emissions are the maximum allowed by the mask. By using both “real” and “ideal” filtering, it is possible to distinguish between the loss of picture quality resulting from the transmitted emissions, and that caused by insufficient receiver selectivity. 1. Or rather that the two signals are being received at the same level. Note that the PAL transmission is characterized by its vision peak sync power, whereas the DVB-T transmission is characterized by the total power of the ensemble. 2. In practice, where the relative power of the PAL signal is as large as this, the DVB-T sideband levels are limited by the need to keep the effective noise degradation (END) of the transmitter within 0.5 dB: high sideband levels in the adjacent channel imply a high equivalent noise floor (ENF) within the “wanted” COFDM ensemble. Ref. [1] provides the relationship between ENF and END. EBU Technical Review - Autumn 1999 2 R. Poole DVB-T There follows a discussion 1. 2. about whether the mask is Define spectral density of the Define PAL receiver filter appropriate, and whether the DVB-T COFDM signal characteristics PAL receiver performance is adequate. As mentioned in the Introduction, practical 3. 4. measurements have been Interpolate to give amount Normalise receiver response, made to establish typical of noise within 100 kHz Define PAL receiver filter ‘buckets’ characteristics receiver performance, and it is interesting to compare these with the predictions of 6. the spreadsheet. Normalize noise power Finally, the article illustrates how problems can arise if the 7. 5. Multiply noise power within Display noise spectrum and spectrum mask is misinter- each bucket by receiver receiver response preted. response 8. 9. The spreadsheet ‘Demodulate’ noise signal by Display noise spectrum and folding lower sideband receiver response design buckets about carrier 10. The design of the spreadsheet 11. is illustrated in Fig. 1. The Apply baseband filtering and weighting, and calculate Display baseband noise numbering of each step corre- picture quality spectra and picture sponds to that of the follow- ing description. Figure 1 1.) The DVB-T spectral den- Flow diagram to illustrate the operation of the spreadsheet. sity is entered in the form shown below (Table 1). The figures in the brown Table 1 bold typeface can be Entry of spectral density data altered at will. Because what is entered is the Assume the spectrum is as follows: noise power, it is also nec- –76.9 dB at –12 MHz and below. essary to specify the bandwidth within which –76.9 dB at –10.75 MHz that power is measured. –76.9 dB at –9.75 MHz Note that – 32.8 dB in –70.9 dB at –5.75 MHz 4 kHz is equivalent to 0 dB within the width of –59.9 dB at –4.685 MHz the ensemble. –56.9 dB at –3.925 MHz –32.8 dB up to –3.9 MHz and above. 2.) The PAL receiver response is entered in a similar manner (see Table (Frequencies relative to centre of DVB-T channel) 2). Note that the response at 0MHz must be given; the other frequencies (in Measurement bandwidth 0.004 MHz brown bold text) can be Level of PAL rel. to DVB-T 0 dB chosen at will. EBU Technical Review - Autumn 1999 3 R. Poole DVB-T Table 2 Entry of receiver response data. PAL receiver characteristics: –47 dB 4.21E–03 (mag) at –1.5 MHz –30 dB 2.98E–02 (mag) at –1.25 MHz –5.5 dB 5.00E–01 (mag) at 0 MHz 0 dB 9.42E–01 (mag) at 1.25 MHz 1 dB 1.06E+00 (mag) at 2 MHz 1 dB 1.06E+00 (mag) at 4 MHz 0 dB 9.42E–01 (mag) at 4.3 MHz –25 dB 5.30E–02 (mag) at 5.5 MHz –25 dB 5.30E–02 (mag) at 6.5 MHz –45 dB 5.30E–03 (mag) at 8 MHz (normalized) (Frequencies are relative to the PAL vision carrier) 3.) The spreadsheet calculates the DVB-T noise power, in dB, that would exist in contiguous 100 kHz “buckets”. It does this by interpolation of the information entered in step 1.). 4.) Similarly, the spreadsheet calculates the receiver gain at 100 kHz intervals. This time, the gain is expressed in linear terms before interpolation is carried out 3 — hence the column of green figures in Table 2. The response is also normalized to 0.5 at the carrier frequency (0MHz). 5.) The DVB-T transmitter noise spectrum (or spectrum mask) and the receiver filter response are plotted on separate charts. The charts provide a useful check that the data have been entered correctly. Examples will be given in the following sections. 6.) The DVB-T noise spectrum is normalized by relating it to the vision peak sync power of the PAL signal — hence the reason for specifying the relative level of PAL in step 1.). 7.) The DVB-T noise power within each bucket is multiplied by the receiver filter response, to give the actual noise power presented to the receiver demodulator.