Ofcom Recommend 64-QAM & 8K-mode for Freeview

Ofcom have published their preferred plan for digital switchover, and they want the current 16-QAM Freeview multiplexes to change to 64-QAM, because this increases the capacity of these multiplexes so that they can either carry more TV channels, increase the quality of existing channels, or allow HDTV channels to be transmitted.

Ofcom also propose that the UK should change over from using 2K-mode DVB-T to 8K-mode DVB-T, with some regions changing early, and the whole of the UK changing by 2012. 8K-mode allows the use of SFNs (single-frequency networks), which allow the available spectrum to be used more efficiently and improve resilience to impulsive interference. SFNs, if planned as such from the outset, allows a single frequency to be used across the whole country, and as there are 46 TV channels that can be used, then this theoretically would allow a very large number of multiplexes to transmit in paralled right across the country. Unfortunately, the UK rushed into using the inferior 2K-mode (if the UK had waite 6 months then we could have used 8K-mode), and the DTT transmitter network could not take advantage of SFNs, and the result is now that people in the UK can only receive 6 DTT multiplexes. Whether this belated change to 8K will allow the UK to use more DTT multiplexes in parallel, or whether the original frequency planning has scuppered the possibility of using more parallel muxes for good remains to be seen. If it is the latter, then this is more proof that the UK's wish to be first with both digital TV and digital radio is about as mis-guided a policy as you can possibly get, because if we are stuck with 6 multiplexes on DTT and are stuck with the dreadful DAB system when many other countries decide against using it, then the combination of these systems will mean that the UK will end up having the worst digital broadcasting implementation in the entire developed world. As Seymour Cray, founder of Cray Computers that manufactured early supercomputers once said "I always want to come second, because that way I let the person that goes first make all the mistakes". 

The moral to the story is: ban government ministers from making any decisions that affect broadcasting systems.

What are 2K and 8K Modes?

2K and 8K refer to the different number of subcarriers that DVB-T uses. DVB-T uses OFDM (orthogonal frequency division multiplex) transmission, which means that thousands of carrier frequencies (referred to as subcarriers) are transmitted in parallel, and so the total data rate is divided amongst the thousands of subcarriers. 

The 2K-mode uses 1705 subcarriers, and 8K-mode uses 6817 subcarriers. The reason why they are referred to as 2K and 8K stems from the fact that the OFDM signal is generated (and demodulated) by a digital signal processing (DSP) operation called the FFT (Fast Fourier Transform), and the FFT requires the number of data values input to the FFT to be an integer power of 2, i.e. 2ⁿ, where n must be an integer. So, 2K-mode uses an FFT with 2048 input data values, and 8K-mode uses an FFT with 8192 input data values. 

For an overview of OFDM, see here, or for a longer, more technical description see here.

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