All posts by Trevor Harris
Below are all of Trevor Harris's postings, with the most recent are at the bottom of the page.
This report makes no mention of sound quality. Not only is DAB greatly inferior to FM many DAB stations only transmit in mono. If one looks at other media such as Itunes we see there is a trend to increase sound quality. They have increased their bitrate from 128kb/s to 256kb/s. The same trend applies to Internet Radio and even the BBC is producing higher quality AAC streams free from flash. Radio 3 is available at 320kb/s in AAC. The BBC has also expressed an interest in surround sound radio something which cannot be supported by DAB.
It has always seem to me that the best way to recieve radio in the house is by wifi. There is a choise of hundreds of thousands of stations from around the world some of them are flac streams which are lossless. They also only need a small aerial.
As for car radios there is 3g and 4g with FM remaining until coverage is adequate.
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The ofcom report does mention error free reception but as any engineer will tell you there is no such thing. All communication systems have a finite error rate all that error correction does is to reduce the probability of an uncorrectable error. As I have pointed out the DAB error correction is very weak so the probability of uncorrected errors are quite high. This is why DAB+ has added an extra layer of error correction. Ofcom are baseing thier study on signal strength alone but signal strength is only one of the factors that determin error rates.
There has never been any sugestion that FM is to be turned off. It is only the National FM stations that Ofcom wants to turn off so the frequencies can be used by other fm radio stations.
Ofcom is clutching at straws if it believes that people most people will want DAB if the coverage was better. The simple fact is FM is better.
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I know my posts have been very negative so let me say something positive.
There are several alternatives to DAB and one I am particularly interested in is Digital Radio Mondial (DRM). This is not a multiplex system so is suited for local radio and can be used on AM and FM frequenies. Infact it can coexist with FM stations in Band II. The BBC has been heavily active in researching DRM but for obvious reasons has been very quiet about it.
It uses AAC and so is very efficient and can be used for surround sound.
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@ Johnathon Green
I am always carefull to say that most people don't want DAB. Some people are perfectly happy with DAB but my point is that the considerable expense of the system cannot be justified for such a small number of listeners.
Sound quality is subjective and some people are more sensitive to mpeg 2 artifacts than others so it might be acceptable to some people.
It is all rather complicated by the fact that the BBC uses Optimod digital sound processors on nearly all of its radio including FM. The manufactures claim that this filter will reduce the effect of mp2. The BBC does not use these horible devices on the 320 kb/s AAC streams.
Car radios with 3G are already available. I agree that coverage is not good enough yet and there are uncertainties about it's viability for radio. Again I think most people are happy with FM for thier car radios. Certainly the BBC will not be able to turn off the national FM stations for a very long time. Most mobile phones have an FM tuner as well.
I don't agree that 128k mp2 is better than FM. Infact the BBC's own research indicated that FM quality was about the same as MP2 at 225kb/s. Originally the BBC used 256kb/s.
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Dab radio uses convolutional coding for error correction which gives unequal error protection to different parts of the audio stream. This is considered a "weak" system because it causes audible "burbling" in poor reception conditions. There is no sharp cut-off but a gradual deteriation. This was fixed in DAB+ by using Read-Solomon as an inner layer of error correction. This is an "equal error protection" which gives a much sharper cut-off. Section 5.27 of the Ofcom report mentions this and that manufactures are trying to make this less noticable. Once a corrupt audio stream has passed through without the error being detected there is no way of knowing it is corrupt. They are trying to make a silk purse out of a pigs ear.
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@Mark
I think you are quite correct here the BBC policy is all about saving money. Unfortunatly the BBC underestimated the cost of DAB. To make things worst Local radio was to be transmitted from commercially funded multiplexes. Well that just ain't going to happen. So Ofcom is now running round like a headless chicken trying to find a solution.
To reach the sort of coverage that Ofcom is aiming for in this report will cost an enormous amount of money. Even if Ofcom do build this system it will be a white elephant as very few people will be using it. There is no way that the BBC are going to be able to turn off the National FM stations with millions of people still listening to FM.
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A national DAB station is much more expensive. These figures are for a coverage of 99% of the population. Ofcom have recently dropped the word population which could mean any thing.
Dab 99%
No of transmitters 1000
Multiplex running cost for 1 year £40M
Cost per annum per DAB station at 128kb/s £5,925,926
FM 99%
No of transmitters 230
Total transmision costs £10,209,00
Cost per network station £2,552,250
Cost per national FM station
The DAB figures are estimated based on the cost of 90% coverage in 2011. The FM costs are actual costs for 2009. The real killer here is the move from 90% to 99% for DAB. DAB has about 230 transmitters for 90% coverage but will need 1000 for 99%.
If Ofcom adopts its new standards as given in the paper the cost of DAB will rise even further.
For those interested for more detail just google
"bbc's efficient and effective use of the spectrum"
This was published in 2007 so is a little dated.
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The BBC has another problem with DAB. It is a single frequency network and so coverage can be limited by self interference. In fact this is already a problem in some areas. The BBC has only made plans for up to 97% coverage and so we don't even know yet if DAB can extend it's coverage to 99%.
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@Dave
Actually Dave you raise an interesting point. The problem we have in this country is that most of the commercial stations sound exactly the same. The reason is that most are run by large commercial groups and are not independent of each other. These groups are only interested in making money and so stick to a formula to do that. In my youth I use to listen to a pirate station called "Radio Jackie". They only ran adverts from local business and all the programs were locally orientated. It was very successfull gaining a very large audience and raising lots of money for local charities. It is legal now after buying up a failing commercial station.
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Saturday 2 July 2011 5:49PM
As for car radios Ofcom believes that cars may be converted to DAB with a simple convertor. If only they would do a bit of research. These devices simply do not work. Infact one manufacturer has abandoned production of these devices because they has a 40% return rate.
Many radios do not have an audio input and so these devices use a "spare" FM radio frequency to transmit to the radio. Unfortunatly in many areas there is no "spare" frequency to use. Certainly this is a problem where I live.
The other problem is that an external aerial is require to get a consistant signal. That will invalidate you warenty.
The plain fact is that most people are perfectly happy with thier FM car radios and do not need to change.