Dab Adaptors

December 21, 2008

Hi folks,

has anyone had experience of DAB adaptors fitted to a 4.3? The most attractive offering at the moment seems to be the "Pure" Highway ( http://www.properprice.co.uk/pure-highway-...CFcse3godVTQwCw ) though there seem to be some doubts about the efficiency of the supplied ariel. One for the electronics guys: Would a tee joint in the existing ariel be any better than the stick on unit supplied with the unit? Any thoughts would be appreciated.

December 21, 2008

If you split the signal, the efficiency of the ariel gets reduced by 50% (6 dB). In other words; you need an ariel that is twice the size for it to add up.

December 21, 2008

If you split the signal, the efficiency of the ariel gets reduced by 50% (6 dB). In other words; you need an ariel that is twice the size for it to add up.

Why would you need to split the signal though??

Would the car aerial fit straight into the DAB unit - it's not like you'd need the original head unit to have an aerial connection anymore !

December 21, 2008

Chris,

Short answer, I would try the stick on aerial first and see how you get on. Maybe find a temporary way of fixing it, to try it out.

Then try the tee and if it is OK for you, then, fine, don't worry (but it is not as simple as that).

Pity, I don't think the Toyota DAB adapter is compatible with the Sat Nav in the XT5, but I may be wrong.

Long answer. Do you really want this? :rolleyes:

Right, St Thicket is correct, but it is not quite that simple. With broadcast reception antennas, you can generally get away with murder because there is usually so much field strength available anyway.

If we were engineering this properly:

First an antenna has to be designed for the frequency (or range of frequencies - bandwidth) it is to operate at. The RAV one will be designed to operate for the FM band centred around 100MHz. DAB in the UK operates around 220MHz-ish. So, to start with it may not be as efficient as it could be. You can get antennas to resonate at more than one band of frequencies, but that is another story (thankfully) and you never know the RAV's might.

From what I can see the RAV antenna is much shorter than the "standard" 1/4 wavelength over a ground plane (vehicle body) which many people may be familiar with. I believe it is a helical antenna, which can shorten the length needed for a given frequency, but will affect it's "pick up" pattern. An antenna (even a simple 1/4 wave) almost never receives equally in every direction. While this antenna may be OK @ 100MHz it could be cr*p at 220MHz.

Secondly, at high frequencies, to get as much signal from the antenna into the receiver as possible, everything has to be what we called "matched". e.g. the antenna has to appear to have an impedance of say 50ohms, the coax has to have what is called a "characteristic impedance" of 50 ohms and the receiver itself has to have an input impedance of 50 ohms. If not, we get reflections in the system and not all of the received power is absorbed in the receiver.

If you then go and change this system, by connecting two receiver, you've changed the match and could well screw the reception on both. If you just had the DAB receiver only attached to the antenna/coax cable, that would be OK, but as I said above the existing antenna may not be as good as it could be - at 200MHz it may not have an impedance of 50 ohms and/or a great "pick up" pattern.

There are ways to spilt the signal and still keep things matched. Simple resistive splitters are lossy:

http://www.microwaves101.com/encyclopedia/...e_splitters.cfm

but there are other methods that are not nearly so lossy.

It is/can be very complicated. I'm trying to simplify it a bit here, but it never that easy to do. Fair to say a lot of Electronics Engineers can struggle with this stuff. I've always had an interest in the radio/high frequencies and over the past few years I've been involved with it professionally too.

Without taking a few measurements, in truth, there is really no clean answer, that's why I say just try it.

The DAB transmission system is very clever though. To cut a long story short, signals from BBC National stations, for example, can be on the same frequency and in fact signals from different transmitters carrying the same programme can "reinforce" rather than interfere as with FM. The downside at the moment is that the "codec" (the thing that compresses the audio like mp3), for the DAB standard is I believe only mpeg2 and with the data rates they have chosen, in some circumstances it can sound worse than FM. I think there is a new standard on the way called DAB+ which improves on this, so you can get higher quality for the same bit rate.

When it was first launched, for many stations, the bitrate was much higher and it sounded probably better than FM. Then they went for quantity not quality, but reducing the bitrate for most stations.

I do go on, don't I...................

December 21, 2008

Chris,
Short answer, I would try the stick on aerial first and see how you get on. Maybe find a temporary way of fixing it, to try it out.
Then try the tee and if it is OK for you, then, fine, don't worry (but it is not as simple as that).
Pity, I don't think the Toyota DAB adapter is compatible with the Sat Nav in the XT5, but I may be wrong.
Long answer. Do you really want this?
Right, St Thicket is correct, but it is not quite that simple. With broadcast reception antennas, you can generally get away with murder because there is usually so much field strength available anyway.
If we were engineering this properly:
First an antenna has to be designed for the frequency (or range of frequencies - bandwidth) it is to operate at. The RAV one will be designed to operate for the FM band centred around 100MHz. DAB in the UK operates around 220MHz-ish. So, to start with it may not be as efficient as it could be. You can get antennas to resonate at more than one band of frequencies, but that is another story (thankfully) and you never know the RAV's might.
From what I can see the RAV antenna is much shorter than the "standard" 1/4 wavelength over a ground plane (vehicle body) which many people may be familiar with. I believe it is a helical antenna, which can shorten the length needed for a given frequency, but will affect it's "pick up" pattern. An antenna (even a simple 1/4 wave) almost never receives equally in every direction. While this antenna may be OK @ 100MHz it could be cr*p at 220MHz.
Secondly, at high frequencies, to get as much signal from the antenna into the receiver as possible, everything has to be what we called "matched". e.g. the antenna has to appear to have an impedance of say 50ohms, the coax has to have what is called a "characteristic impedance" of 50 ohms and the receiver itself has to have an input impedance of 50 ohms. If not, we get reflections in the system and not all of the received power is absorbed in the receiver.
If you then go and change this system, by connecting two receiver, you've changed the match and could well screw the reception on both. If you just had the DAB receiver only attached to the antenna/coax cable, that would be OK, but as I said above the existing antenna may not be as good as it could be - at 200MHz it may not have an impedance of 50 ohms and/or a great "pick up" pattern.
There are ways to spilt the signal and still keep things matched. Simple resistive splitters are lossy:
http://www.microwaves101.com/encyclopedia/...e_splitters.cfm
but there are other methods that are not nearly so lossy.
It is/can be very complicated. I'm trying to simplify it a bit here, but it never that easy to do. Fair to say a lot of Electronics Engineers can struggle with this stuff. I've always had an interest in the radio/high frequencies and over the past few years I've been involved with it professionally too.
Without taking a few measurements, in truth, there is really no clean answer, that's why I say just try it.
The DAB transmission system is very clever though. To cut a long story short, signals from BBC National stations, for example, can be on the same frequency and in fact signals from different transmitters carrying the same programme can "reinforce" rather than interfere as with FM. The downside at the moment is that the "codec" (the thing that compresses the audio like mp3), for the DAB standard is I believe only mpeg2 and with the data rates they have chosen, in some circumstances it can sound worse than FM. I think there is a new standard on the way called DAB+ which improves on this, so you can get higher quality for the same bit rate.
When it was first launched, for many stations, the bitrate was much higher and it sounded probably better than FM. Then they went for quantity not quality, but reducing the bitrate for most stations.
I do go on, don't I...................

WOW! Thanks a lot fella. Just the sort of answer we have come to expect from you - comprehensive but understandable ( and facinating! ). If you were doing this would you wait for DAB+ before making a move? I guess that there is going to be a market for this sort of thing for some years so they could both improve and get cheaper.

I have to admit that I'm pretty disillusioned with the current BBC output other than R7 which is mostly repeats from my youth ( well the older bits are ), which I'm afraid I rather enjoy.

Many thanks to you other guys for your advice as well. :thumbsup:

December 21, 2008

I fitted one of these in my sporty a couple or so months ago. I experimented with the screen mount aerial that came with the highway and managed to find that the best position was centre of the screen behind the mirror pointing down. But even in this position I was regularly loosing signal as I drove round town.

Eventually I bought the official external aerial (http://www.pureservicecentre.co.uk/acatalog/Aerials.html) and have never looked back since. There are a couple of "black spots" where I totally loose signal, but this is when I am usually adjacent to very tall buildings.

Overall I am very impressed with the Highway and the external aerial.

:)

December 21, 2008

Never afraid to ask the dozy question.

Along with the digital changeover for TV is the same then happening for radio?

If so I'm assuming we shall all need adapters. I'm assuming they plug into the existing units.

December 21, 2008

Along with the digital changeover for TV is the same then happening for radio?

AFAIK No, not yet. These things get reviewed from time to time. Ofcom's site suggests next review for FM is 2012. So not for a few years after that at the earliest I expect.

Chris - Summary on DAB+:

http://www.which.co.uk/advice/dab-plus-explained/index.jsp

Looks like malta have adopted DAB+

http://www.bbc.co.uk/pressoffice/pressrele.../01/malta.shtml

EDIT:

Or maybe by 2017 :rolleyes: :

http://news.bbc.co.uk/1/hi/entertainment/7792083.stm