Lecture Series in Mobile Telecommunications and Networks (1583KB)

Questions & Answers
Michael Walker: Thank you, Peter, for that splendid talk. Peter has agreed to take questions, so we have the
opportunity to tease out more information.
John Lowe (Institution of Mechanical Engineers): Thank you, Peter – it is a fascinating future that you have
described. What worries me is the duplication of standards between the three territories of the Far East, Europe and
North America. Could you fill us in on the prospect of some compatibility or singularity of direction?
Peter Vary: Yes, that is a very good question. We have many standards – perhaps too many – but this is also evolution.
We are in the happy situation that, in the meantime, we have worldwide agreement on two sides. One is the ITU
(International Telecommunications Union), which is more or less responsible for fixed line, voice-over IP business.
On the other side we have what is called the 3GPP (Third Generation Partnership Programme). They talk to each other
and take over the standards we have for cellular radio meanwhile in the ITU world. Things are getting better.
There will be many different standards but there will be a convergence, I am pretty sure but, for reasons of compatibility,
you will need to have the old codecs. I have a collection of some old phones – my first one is the Motorola shown on
the slide, and it still works. If I switch it on, it says that it is no longer compatible with the system, but it works. We have
to keep the compatibility but, on the other side, it is not too expensive because it is just software – but, saying that it is
‘just software’ means that most of the signalling processing is carried out on a programmable signal processor in the
mobile phone. You have a lot of memory there and selecting a different codec is just a matter of selecting a different
part of the memory where you have the programme for the codec A, B, C, D, E. It is not that difficult, but it is more
difficult than in the old world where you just had one or two different versions of A-law PCM and Mu-law PCM. It is
becoming more difficult but it is not too expensive.
Professor Ralph Benjamin (Visiting Professor, Bristol University): Right at the beginning, you mentioned the
importance of understanding the physiological constraints on speech generation as a guide to the optimum use of a
limited number of bits in the information, which will probably be even more important in speech extension, bandwidth
expansion, or in turbo decoding. However, this is only one of probably three separate elements because you also have
the constraints of language and dialect. In American southern states, people talk with a long drawl; other languages are
totally different from English and, in one extreme, some African languages consist of a lot of staccato clicks. The features
you have to encode depend rather critically on this.
A third feature might be that different elements in your speech may be of different sensitivity in terms of recognition
and understanding, or acceptability by the user. You may need rather a clever combination of those three features to
make the best use of the information in encoding in the first place, or in knowing which preamble or supporting
parameters are required to tell you how to do your bandwidth extension. Thank you.
Peter Vary: The standardisation of the different speech codecs is always based on subjective listening tests. In the
codec competition, different companies are proposing codec candidates and there is no objective measure that is so
reliable that you can rely on the objective measure. Therefore, we need subjective listening tests, which are very
expensive, in different languages, with native listeners and native speakers. That, first of all, is a necessary constraint
which is fulfilled for the coding part.
For bandwidth extension, I can tell you that it makes a big difference to apply bandwidth extension for German or
French or Japanese. You hear the variation and it might work better for one language than for another. You could
implement a language switch, or language recognition, or something like that. That is why the artificial bandwidth
extension without any side information is only the second best solution: it gives you some wideband impression and
sometimes it works well. If you could adapt it to the speaker, then it would be very nice.
Therefore, the proposal to add side information – in that case, the concept will hide these bits in the bit-stream, but we
first have to get the bits. This is done in such a way that, at the transmit side, we have the higher frequency band and
then we apply there locally the artificial bandwidth extension and we compare it with the original. We then transmit
the correction terms as hidden information to the receiver. If you have a strange language there, hopefully the
56 The Royal Academy of Engineering