13 replies to this topic

[MLXXX]

A current thread, Transmission Coverage Of Qpsk/qam64/qam256, has made reference to variations in coverage for digital television broadcasting using different modulation constellations. The more complex modulations require a higher carrier to noise level ratio at the reception location to achieve an acceptably low Bit Error Rate in the receiver. The more complex modulations have a smaller coverage area unless the Effective Radiated Power is boosted.

Similar variations in broadcasting coverage area arise with Digital Radio Mondiale. A more complex modulation constellation (64-QAM) requires a higher Effective Radiated Power if it is to achieve a similar coverage area to a less complex modulation constellation (16-QAM). [For a description of Quadrature Amplitude Modulation, see http://en.wikipedia....tude_modulation.]

DRM30 (Digital Radio Mondiale for bands below 30MHz) has been proposed for consideration as a possible standard for use in local regional digital radio broadcasting in Australia. (It is a different standard to the DAB+ standard already in use in certain metropolitan areas in Australia).


DRM30

With DRM30 the main service channel may be modulated at 64-QAM or 16-QAM. Nominal bandwidths of 9kHz or 10Khz are usual (although 4.5kHz, 5kHz, 18kHz and 20kHz are also included in the standard). Other parameters are also defined ('robustness", and "protection level").


Table A1.1 on page 69 of EBU Tech 3330 is headed:

S/N (dB) to achieve BER of 1 x 10^–4 for all DRM robustness modes with spectrum occupancy types 2 or 3 (9 or 10 kHz) dependent on modulation scheme and protection level for channel model No. 1

Note: Channel model No. 1 “represents the typical behaviour of a transmission channel with ground-wave propagation during daytime in LF and MF bands”.

The table includes the following figures for a nominal transmission bandwidth of 9kHz:

16-QAM (with protection level 0): 8.6 dB

64-QAM (with protection level 0): 14.1 dB

64-QAM (with protection level 1): 15.3dB

It will be noted that the difference between using 16-QAM and 64-QAM (if both use protection level 0) is 14.1 – 8.6 = 5.5dB.

The difference between 16-QAM (with protection level 0) and 64-QAM (with protection level 1) is 15.3 – 8.6 = 6.7 dB.

A field trial in Spain found somewhat smaller differences. See page Table II on page 3 of Minimum C/N Requirements for DRM Reception based on Field Trials which shows figures of:

16-QAM (with protection level 0): 10.2 dB

64-QAM (with protection level 1): 15.0dB

This is a difference of 15.0 – 10.2 = 4.8 dB.

4.8dB corresponds to a power ratio of 3:1.  So, for example, based on the particular field trial, if a 5kW transmitter were needed using 16-QAM, a 15kW transmitter would be needed for a similar coverage area using 64-QAM.


DRM+

With DRM+, 16-QAM and 4-QAM are defined. A field test (see page 15 of Preliminary report : DRM+ measurementsin band II) found these results:

Tests were made at a frequency of 95.2 MHz in urban surroundings and on a radial route passing through the city of Hanover and rural environments in the main beam of the transmission. Measurements of the fieldstrength, the bit error rate, the calculated signal to noise ratio and the audio status show that in 4-QAM mode with a coderate of 0.33 reception with good audio quality was possible down to a fieldstrength of around 30 dBµV/m and a calculated SNR of 10 dB. In the 16-QAM mode reception was possible down to 46 dBµV/m at an SNR of around 18 dB.

So in that field test, the difference between using 16-QAM and 4-QAM corresponded to a difference in SNR of about 18 – 10 = 8dB.

Based on this field test, a DRM+ transmitter would need to use 8dB more power, if using 16-QAM rather than 4-QAM. 8dB corresponds to a power ratio of 6.3:1. An online conversion tool for decibels to power or voltage ratios can be found at http://www.mogami.com/e/cad/db.html


Comment

Implementations of DRM30 allow a wide range of choice of transmission settings, not only modulation constellation, but “robustness” and “protection level”. These all influence the coverage area that can be achieved.

The audio bitrate that can be supported for the main service channel varies widely depending on these settings. Settings that increase coverage area reduce the audio bitrate available.

For a general and relatively non-technical presentation of DRM30 protection modes, and some of the corresponding audio bitrates,* see the webpage http://www.drmradio.co.uk/modes.html


________________________________________

EDIT: The following footnote to the last paragraph above does not relate to coverage, but to audio quality.

* Assessments of the perceived audio quality of audio codecs at low bitrates vary greatly between human listeners. However in my opinion the following  part of the webpage may err on the generous side in its descrption 'near FM quality':

Only the higher bit rates available in Mode A and Mode B could be described as ‘near FM quality’ - the main selling point of DRM. However to achieve these higher bit rates the protection ratio has to be kept low so the received signal is more susceptible to interference or fading.

The table for a 9kHz transmission bandwidth shows a maximum bitrate of 30.8kbps. If this modest biitrate were used for non-parametric stereo in conjunction with the AAC+ codec, the resulting sound quality would be quite "ordinary" for many listeners. Whether it would actually reach the standard of 'near FM quality' [whatever that means exactly] for some 'hypothetical average listener' could be debated.

A nominal 18kHz radio frequency bandwidth could double the available audio bitrate. However, world implementaions of DRM to date do not -- to the best of my knowledge -- use such a high bandwidth.

Edited by MLXXX, 19 August 2012 - 10:52 AM.

[alanh]

MLXXX,
This backs up what I have said about QPSK and QAM64 for Brisbane community and SBS TV.
Funny how I was so wrong

AlanH

[MLXXX]

If it matters, I would say you were wrong (about the relative coverage areas of 64-QAM and QPSK) until quite recently.  In the recent thread Sharp Tvs Crippled By 31 Digital you suggested 64-QAM and QPSK have the same coverage area, differing only in reliability of reception at the outer edges of the coverage area. However subsequently in the opening post of Transmission Coverage Of Qpsk/qam64/qam256, you indicated you had been "far too generous". At post #6 of that thread I congratulated you for that clarification of your position.

.

[DrP]

I'm still waiting for an 'expert' to tell me if SBS and Bris31 both use QAM64 or if they both use QPSK.  I do wish he'd  speak up and clarify that point.

[The Baja]

Plagiarism

A lot of it going on, We can ALL do Google Search and try to make ourselfs look GOOD on this FORUM. Think about It & Give due reference if the work is NOT your OWN.


Cheer's



The Baja

[MLXXX]

The Baja,
do you consider my post 1 insufficiently referenced to sources? It already contains many references (8 in all, as hyperlinks). The hyperlinks, which  I inserted when I created the post, take you to relevant sources.

You may not quite understand the context for this thread. Alanh had been maintaining there was no difference in coverage area as between 64-QAM and QPSK in Australian DVB-t broadcasting.  This thread was able to refer to field testing done in Spain and Germany in relation to DRM and DRM+, which involve comparable choices in modulation constellations. The field testing confirmed there are significant differences in required S/N, for low error rate reception.

What we tend to find on this forum from alanh are bald claims with no references at all. We query him. He then often asks us to disprove his unreferenced claims. * SIGH *

Edited by MLXXX, 24 August 2012 - 01:51 AM.

[GoForMoe]

Do you know much about the protection level options on DAB+ - what was the basis for picking Protection Level 3? Could we ever see DAB+ in Australia utilise the higher bandwidth options?

If I'm not mistaken it's also an option to use different protection levels on different services - would it be feasible to for example put the ABC Local Radio station on the highest protection level to get that station out as far as possible, while putting some of the digital only stations on a lower protection to give them better sound quality?

[MLXXX]

GoForMoe, on 25 August 2012 - 01:54 AM, said:

Do you know much about the protection level options on DAB+ - what was the basis for picking Protection Level 3? Could we ever see DAB+ in Australia utilise the higher bandwidth options?

Protection level 3 supports mobile use (car, public transport). Here's a description about a DAB ensemble, but I believe it is the same basis for DAB+ (from 5.3.3.3 Protection Levels at page 31 of ETSI TR 101 496-3 V1.1.1 (2000-11) ):-

For the compilation of a DAB multiplex, a reasonable trade-off between the number of programmes, the audio data rate,

and the error protection level has to be made. The benefit of the high performance protection levels is that the samples

are protected very well and that the curves of the residual bit error ratio versus C/I become steeper. In high speed,

mobile, reception an error floor exists for protection class 3 at protection levels 4 and higher. This situation can easily

be improved by using the next highest protection level. Protection level 4 (mobile weak) may be used when the service

is not addressed to mobile receivers.

I'd assume the Australian broadcasters would not want to put reception in a car in jepoardy. By the way I think the existing transmissions could allocate up to 192kbps to an individual station in the ensemble, based on use of 144 Capacity Units (though I believe the highest we've seen so far has been 128kbps). See the top paragraph of page 11 of AS 4943-1-2009 Digital radio—Terrestrial broadcasting Part 1 Characteristics of terrestrial digital audio broadcasting (T-DAB+) transmissions (e.g. at http://www.docin.com/p-46411203.html).

GoForMoe, on 25 August 2012 - 01:54 AM, said:

If I'm not mistaken it's also an option to use different protection levels on different services - would it be feasible to for example put the ABC Local Radio station on the highest protection level to get that station out as far as possible, while putting some of the digital only stations on a lower protection to give them better sound quality?

I'm not sure about that. Perhaps someone with a more detailed knowledge could comment.

[Malich]

MLXXX, on 25 August 2012 - 12:59 PM, said:

GoForMoe, on 25 August 2012 - 01:54 AM, said:

If I'm not mistaken it's also an option to use different protection levels on different services - would it be feasible to for example put the ABC Local Radio station on the highest protection level to get that station out as far as possible, while putting some of the digital only stations on a lower protection to give them better sound quality?

I'm not sure about that. Perhaps someone with a more detailed knowledge could comment.

Yeah, the protection levels you're talking about there are per-service, not per-ensemble.

As for "get that station out as far as possible", that's travelling dangerously close to QPSK vs 64QAM territory . AFAIK it should work as you say, but I've no idea how much would be actually gained by doing so.

[GoForMoe]

I think the main reason would be the numerous regional situations of stations with varying licence areas in the same broad geographic area. On DRM you could easily set up both the transmitter network and the modulation to get each station to cover its relevant area - but on DAB with its frequency block model, the spectrum isn't there (or at least it's not an efficient usage of it) to use multiple multiplexes.

I'll use a few examples - for one, Geelong - it's a regional market with only two commercial stations and two community ones and no ABC local radio. In this case using a higher protection level on each of the services allow a more stable signal and lower power demands for the coverage area (which will help the economics of DAB).

Secondly, there's the Maryborough licence area - with the AM 'Easy Mix' (3EL) as its only station. It has a coverage area that mostly overlaps with Bendigo such that it gets a Bendigo City FM repeater, resulting in a quasi third commercial broadcaster in the area. So how would you achieve this with DAB? A separate Maryborough DAB multiplex to cover the specific licence area?

I've mapped this - the red is the Bendigo RA1 Licence area, the blue is the Maryborough RA1 licence area and the purple is the overlap.


I'd think that if the station was put on Bendigo DAB, but with a lower protection level, it would cover the main parts of its licence area, but have a weaker signal out into the Bendigo only part of the licence area.

[alanh]

MLXXX,
Stop blaming me for lack of references. I give links to the original content and not copies of an original if possible. I have even been criticisised  for not making comments, because I want posters to make their own conclusions.

The material which has no links is my original work as shown in the diagrams in the attachment strand. I use my considerable technical knowledge and experience to produce information which applies to the topic.

I will not give references to posts where I have been given information which has been provided by industry insiders, to preserve their anoniminity. They still need to keep their jobs.

AlanH

[James T Kirk]

alanh, on 30 August 2012 - 08:49 PM, said:

I will not give references to posts where I have been given information which has been provided by industry insiders, to preserve their anoniminity. They still need to keep their jobs.

AlanH

For once I agree with AlanH, any industry insider who is found to associate with AlanH is at risk of losing their job.

AlanH is a master poster not a master of the subject matter.

James

[MLXXX]

alanh, on 30 August 2012 - 08:49 PM, said:

I have even been criticisised  for not making comments, because I want posters to make their own conclusions.

Of course you have been criticised.

It is indeed poor forum etiquette to post a bare link. One is expected to start a discussion. That's what forums are for. They are not a news digest service of new topics with bare links.

Quote

The material which has no links is my original work as shown in the diagrams in the attachment strand. I use my considerable technical knowledge and experience to produce information which applies to the topic.

You appear to be referring to your occasional self-authored pdfs.

Quote

I will not give references to posts where I have been given information which has been provided by industry insiders, to preserve their anoniminity. They still need to keep their jobs.

You may need to choose more reliable "insiders", alanh.  So much of the information you provide to this forum turns out to be wrong or misleading.

An example of the sort of claim that I say is unreferenced is your astonishing claim that NVIS would be viable for local broadcasting in the 26MHz band. There is no support anywhere on the web for that odd claim. A more recent example is your amazing claim that QPSK and 64-QAM have the same coverage area differing only in reliablity at the outer edges of the coverage area.

It's these sorts of imaginings of yours for which I (and others) have sought references. You have provided none.  You have withdrawn your claim about QPSK and 64-QAM. You have not withdrawn your claim about using NVIS in the 26MHz band as an ongoing local broadcasting service.

If these sorts of imaginings are not your own but claims of "insiders",  may I suggest you check with or change your sources? Some of your insider contacts appear to have been misleading you.

Edited by MLXXX, 31 August 2012 - 09:04 AM.

[DrP]

Even when alanh seems to have the inside line what he posts often has little fidelity with what he was told.  The DAB+ infill repeater debacle is a perfect example.