Differences between DVB-T radio and DAB
Bob
Apologies for asking as this has probably been discussed many times,
but what are the major differences between DAB and radio over DVB_T? I
have a set-top box and the sound from the radio stations is perfectly
fine. It is a lot better than DAB and I pick up a lot more stations
than DAB. Do DAB and DVB-T share the same frequencies/multiplexes? Is
it to do with mobile transmission and reception.
What modulation systems to DVB-T use? Is it QAM compared to DQPSK for
DAB, so would QAM not allow more information to be transmitted?
I've also read that DVB-T also has RS error correction which DAB
hasn't, so what is the real point in going to DAB+ ? It seems it is
really due to the Audio codec. Would it not be better to use the HE-
AAC codec with the DVB-T system.
So, I'm really just wondering what is the point of DAB and DAB+ when a
standard like DVB-T already exists.
Regards
Bob
Mark Carver
> Hi,
>
> Apologies for asking as this has probably been discussed many times,
> but what are the major differences between DAB and radio over DVB_T? I
> have a set-top box and the sound from the radio stations is perfectly
> fine. It is a lot better than DAB and I pick up a lot more stations
> than DAB. Do DAB and DVB-T share the same frequencies/multiplexes?
No, in the UK DVB-T is broadcast in UHF bands IV and V (470 to 860 MHz), DAB
is broadcast in VHF Band III (174 to 230 MHz). DVB-T's transmission
characteristics have been designed around the same concept for analogue TV,
fixed roof-top receiving aerials. DAB has been designed for mobile reception,
both use COFDM modulation, but the mode settings for DAB are far more robust
(resulting of course in a lower data 'payload' typically 1.3 Mb/s per mux ).
Radio on DVB-T simply shares the space on the six national TV muxes.
> What modulation systems to DVB-T use? Is it QAM compared to DQPSK for
> DAB, so would QAM not allow more information to be transmitted?
Yes, UK DVB-T uses 16QAM for four of the muxes (giving a payload of 18 Mb/s
per mux), and the other two 64QAM giving 24 Mb/s per mux.
> So, I'm really just wondering what is the point of DAB and DAB+ when a
> standard like DVB-T already exists.
See above, the parameters of UK DVB-T are not suited to mobile reception.
--
Mark
Please replace invalid and invalid with gmx and net to reply.
Richard Evans
> Hi,
>
> Apologies for asking as this has probably been discussed many times,
> but what are the major differences between DAB and radio over DVB_T? I
> have a set-top box and the sound from the radio stations is perfectly
> fine.
more capacity available, although this is no always the case. So sound
quality on DVB-T will tend to be better than on DAB because of the
higher bit rates. Although personally I find DVB-T radio usually doesn't
sound very nice to listen to when played through a hi-fi system.
Unfortunately sound quality seems to be a very low priority for the
broadcasters.
> It is a lot better than DAB and I pick up a lot more stations
> than DAB. Do DAB and DVB-T share the same frequencies/multiplexes?
No.
DVB-T uses channels in the UHF TV band. Each multiplex using the same
bandwidth as one TV channel.
DAB uses Band III VHF, using part of what was Band III TV spectrum. Each
DAB multiplex uses 1/4 of what used to be a Band III TV channel.
> Is
> it to do with mobile transmission and reception.
DVB-T is designed to be received by rooftop TV aerials and is not
designed for mobile reception.
DAB is designed for mobile reception using simple aerials. Actually it
was specifically designed for in car reception.
>
> What modulation systems to DVB-T use? Is it QAM compared to DQPSK for
> DAB,
At the moment 4 multiplexes use 16 QAM and 2 use 64 QAM. It is planned
that after analogue switch off all multiplexes will use 64 QAM, except
for the HD multiplex which will use a new standard, possibly with 256 QAM.
> so would QAM not allow more information to be transmitted?
Yes. Twice as much with 16 QAM, 3 times as much with 64 QAM, 4 times as
much with 256 QAM.
>
> I've also read that DVB-T also has RS error correction which DAB
> hasn't,
I don't know what error correction DVB-T uses, but it probably would
include RS Coding. DAB does not use RS Coding, but it has been added for
DAB+, to make it more robust than DAB.
> so what is the real point in going to DAB+ ? It seems it is
> really due to the Audio codec.
AAC+ would allow the same audio quality as DAB with about 1/3 of the bit
rate. That is rather a big advantage. DAB+ also has RS Coding in
addition to the convolutional error correction that DAB has. This can be
used to increase capacity by around about an extra 30%. Reducing the
convolutional error correction from level 3 to level 4, adds 50%
capacity, but with less protection. RS coding makes up for this adding
extra robustness, but using less bits, giving an over all gain of about 30%.
> Would it not be better to use the HE-
> AAC codec with the DVB-T system.
DVB-T is now the old standard. There is a now new standard DVB-T2 which
I pretty certain does use aac+. (aac+ is a more general name for HE-AAC
and and newer version that also allows parametric stereo).
>
> So, I'm really just wondering what is the point of DAB and DAB+ when a
> standard like DVB-T already exists.
DVB-T is not designed for mobile reception. You can only receive it
mobile when you are in a rather good signal area. DAB is far more robust
when mobile than DVB-T. There are however other versions of DVB that are
designed for mobile reception DVB-H and DVB-H2. DVB-H2 would be a very
good standard for digital radio, but it is probably a bit too late in
coming as DAB/DAB+ has already become too well established. Perhaps one
day we may get DVB-H2, or something similar, but it's not likely in the
near future.
Richard E.