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Sunday, 26 May 2013

VHF and UHF digital voice types

This post is aimed at new users of radio, who are interested in digital voice on UHF and VHF. Digital is used mostly by commercial users at this stage, and there are a limited number of digital products on sale for private individuals without getting a business radio license. I hope this comprehensive and extensive article will clear up any confusion for newbies to the field, and also give out new knowledge to the same group.

One product that can be used without a license is the Icom IC-F4029SDR. This is a digital handheld that works on the PMR446 frequencies and on the digital allocation which is just above the normal analogue allocation. These radios currently cost in the range of £200 each, so as you can see, it is an expensive option for communications where analogue radios which would achieve similar range can be bought for £40.
This Icom radio uses 6.25KHz wide channels, and adheres to the dPMR446 standard.(more info here)

Terms you will need to know:
Timeslot: When the channel is divided into segments by time. Data is transmitted in bursts. The switching is done at a rate that it is fast enough not to disrupt the users communications - many times each second.
An example:
A user wants to transmit their digital voice to someone else. They need a certain amount of spectrum to transmit the constant stream of data which is produced by the vocoder. The amount of data per second they need to send is constant but there are different methods of sending it:
They could use 6.25KHz and occupy all of the time on the channel.
They could use 12.5KHz and send the data in half the time, but they need the data sent constantly (because it is a stream of live voice), so the channel is split up into 2 timeslots and the radio transmits the data at twice the rate it would when using 6.25KHz, but the quick bursts that only occupy half of the total time.(1/2 timeslots)
They could also use 25KHz, like tetra, and use only 1 timeslot out of 4. All of these methods allow the same amount of data to be sent, but it different ways. If you still don't understand timeslots, have a look at the article on wikipedia about TDMA (Time division multiple access)
Bandwidth: How wide a channel is. The wider a channel is, the more spectrum it takes, up, and the more data it can be used to transfer. Also license fees increase for wider channels. For comparison, 12.5KHz is a standard analogue FM voice channel; 200KHz is approximately what an FM broadcast station would use; and TV channels are about 8MHz wide (1MHz=1000Hkz);  GSM (mobile phone standard) channels are 200KHz wide and have 8 timeslots.

For commercial users there are quite a few options:
DMR - The most popular mode by far, equipment is sold by Motorola as MOTOTRBO, and also by Hytera and other manufacturers. This standard uses 12.5KHz wide channels and fits 2 timeslots in each channel.(This is called TDMA)  A single repeater can repeat 2 voice conversations (1 way, not duplex) at the same time on the same channel by the timeslots. The vocoder used here is AMBE+2.


TETRA- This is the second most common option. Tetra radios are usually more expensive than the other 2 types discussed in this section, but also have better features. Tetra radios are definitely more advanced than the other two, since they provide functions such as duplex calls (listening and talking at the same time, like a mobile phone); call handover, which is where individual radios are transferred between base stations without losing connectivity so they can continue transmitting or receiving while moving; and DMO gateways, which is where one radio can repeat a signal from the base station to allow another radio that is further away and out of range to connect. Tetra also uses a different vocoder - ACELP. This vocoder in my opinion sounds a lot better than AMBE. Tetra does not often have a range as good as the other two, but the advanced features make up for it. 25KHz channels are used, but in terms of bandwidth per voice channel, it is just as efficient, because 4 timeslots are used. Tetra is usually only used by larger companies or organisations, most likely because of the cost of the base stations, which are far more than DMR or dPMR.
The emergency services (Police, Fire, Ambulance) throughout the UK use tetra on the airwave network which they lease off a private company. This network is fully encrypted and closed to the public.
Manufacturers include Sepura, Motorola, EADS.

dPMR - Equipment sold by Icom and other manufacturers. This standard also uses the same AMBE+2 vocoder as DMR, but this time uses half the bandwidth (6.25KHz) and does not use timeslots.(This is called FDMA) The consequence of this is that each repeater can only repeat one voice at a time, but this also means you do not need a whole 12.5KHz channel and can license a 6.25KHz channel instead, which is cheaper.
The range of DMR and dPMR are similar, but dPMR probably has just a tiny bitof an advantage because it does not use the timeslots and uses only 6.25KHz which makes the radio transmit the same amount of power but in a smaller space.
There is also a very similar standard which is not interoperable with this one, called NXDN. This is manufactured by Icom and Kenwood. Icom make radios that work with both NXDN and dPMR.

Other modes you may hear of:
P25 - Used by the police in America. Scanners are available that can listen to this mode, unlike all of the other modes mentioned in this post. There are two types available: Phase 1, which uses the older and inferior IMBE vocoder and 12.5KHz bandwidth with no time division, and Phase 2 which uses AMBE, and 12.5KHz 2 timeslot division.(similar to DMR)

D-STAR - This mode is used by amateur radio operators. It is omptimised for amateur radio and would be very difficult to adapt to any other type of user. Repeaters require callsigns to be used and sound quality is not that great even though it uses the AMBE+ vocoder. Equipment is kept expensive by the fact that there is no competition to the only manufacturer of the standard - Icom.
It is also unpopular with some users due to the fact that it uses the AMBE+ vocoder which is strictly protected by patents and copyrights, stopping amateur users from experimenting with the vocoder.

Why use digital?
Digital is popular with commercial users who are updating their radio systems because of a few factors:
  • It gives consistent sound quality right up to the fringe of communication range. This means no hiss and noise on the radio, but the downside is that it makes it harder to tell how good reception is, and whether you are about to go out of range. When someone is about to go out of range, digital errors will be present, and this will cause the vocoder to make strange noises, sometimes compared to the little robot, R2D2! This would only happen when analogue voice would be too quiet and noisy to understand.
  • Digital can send voice and data. Users can have their ID show up on other radios when they talk; status messages can be sent; text messages can be sent; emergency priority can be turned on which speaks over other users in an emergency.
  • Because digital uses a vocoder instead of sending the voice using analogue, even if you speak into the radio very quietly or from far away, the message will not be overcome by hiss on the receiving end because there is none!
  • Digital voice is much easier to secure than analogue. Many digital technologies come with basic security and the digital stream can simply be encrypted, which is far easier than for analogue, where it would have to be inverted which is not very secure. Scrambling on analogue loses voice quality, whereas it does not on digital.

2 comments:

  1. Some digital technologies are great for security purpose. Can you please give some more idea about DMR radio? I just have basic idea about DMR radio.

    ReplyDelete
    Replies
    1. DMR is a standard made by ETSI and now used all over the world. It is most commonly know via the MOTOTRBO product line which uses tier 2 DMR. There is a comprehensive wikipedia article on it here: http://en.wikipedia.org/wiki/Digital_Mobile_Radio

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