Trip to HK & CN 2014 – Part 3: Hong Kong Free-to-Air DTMB Analysis

What does somebody do when they’re in a room and want to relax for a bit? They watch television, of course! Having lived the majority of my life in Australia, it also provides the double-benefit of allowing me to soak up a little of the language and culture of Hong Kong. Flicking the TV on in the Harbourview Hotel room wasn’t particularly inspiring – everything’s analog and fuzzy … hmm. That’s not right!

Of course, with me, the situation is never quite that simple. Television is, for all intents and purposes, just another radio signal which begs to be captured, decoded and analyzed.

Television Signals in Hong Kong

Hong Kong is in a state of transition in respect to television signals, similar to where we were a few years back. In Hong Kong, a 625-line analog system remains in use until the end of this year, when it will be switched off, officially marking the completion of the transition to digital TV. This will mean that many viewers will need to upgrade to a TV with an integrated digital tuner, or utilize an external digital set top box. Due to the complications of highrise buildings and shared master antenna systems, some users may have problems even after purchasing the necessary equipment, especially those using analog modulators and closed-circuit TV systems.

While not required to do so, Hong Kong adopted the China-backed Digital Terrestrial Media Broadcast (DTMB, formerly known as DMB-T or CMMB) standard for broadcasting using 8Mhz wide channels, making it incompatible with receivers based on the European DVB-T standard (such as used in Australia) as well as the Japanese ISDB-T standard, and the US ATSC standard. This makes its reception rather interesting to me, but also requires acquiring some equipment to make it happen.

While the DTMB standard isn’t particularly popular, the standard itself is very recent and incorporates many physical layer coding techniques which are superior to those employed by virtually all competing standards. Features such as LDPC error correction coding only are beginning to be implemented by 2nd-generation DVB-T2 standards are standard in DTMB. Also, rather interestingly, DTMB also uses time and frequency domain techniques to improve synchronization speed and channel estimation which makes mobile and handheld reception more likely.

At the moment, the free-to-air television broadcasts are run by three main companies, Television Broadcasts Limited or better known as TVB, Asia Television or better known as ATV, and Radio Television Hong Kong, better known as RTHK. The first two are commercial broadcasters, with the latter being an independent government department, sort of like how the ABC operates in Australia.

Surprisingly, despite there being these free to air broadcasters, the amount of people actually tuning into free to air broadcasts is rather limited, as the programming (at least, according to the locals) is rather thin and poor and the vast majority of serious TV watchers have Cable TV Hong Kong.

The signal frequencies themselves can be found in a document from Office of the Communications Authority, which I stumbled on in a Google search. The terrain in Hong Kong is very hilly and full of urban canyons formed by skyscrapers, which makes it a very difficult and demanding setting for reliable television broadcast. As a result, they have many transmitters scattered everywhere, with a mixture of polarizations and powers, some down to 0.25w – walkie talkie power!

The digital broadcasts are less confusing, with three of the four multiplexes opting to go for a Single Frequency Network arrangement, where all transmitters are sending the same signal at the same point in time, using the same frequency. This has a lot of technical challenges, especially when signals overlap, as they can do so destructively and cause a loss of reception – it is something we have avoided here, although not entirely.

The excerpt for the first page of digital frequencies is shown below. At our very central location, our signal was primarily coming from the Temple Hill transmitter, which is the first row, very conveniently.

DTTFrequencies

You might have noticed that the RTHK column has a lot of shaded cells – at the moment, RTHK is still in the process of building out its transmission facilities and is in a trial of digital broadcasting. Their present list of transmitters covers the approximate area in the diagram (excerpted from their site):

station_map_s

Of course, I didn’t actually know all of this at the time, and instead had to spend time with a tuner and doing some investigation online to find it all out.

Getting a Tuner

Given that I couldn’t really walk much, or if at all, I had to send the family friend on a mission to buy me a tuner. I made it clear that I was after a DTMB digital USB TV tuner, to use with my laptop. I didn’t really need to think about analog – it features less channels, is  on the way out, and is much more difficult to capture, store and process.

Surprisingly, it wasn’t as easy as expected, and in the end there was only a very limited number of stores stocking only two models. That’s what happens when you have a unique standard, and few people actually tuning in.

I ended up with a MagicPro ProHDTV Mini 2. The device is worth HK$270, or about AU$45, but the shop would only negotiate down to HK$320, or about AU$53. Not too bad, given the receipt states the discount was from HK$750. Yikes! The tuner itself was taken out of the box at the time I took the shot.

20141231_165200

It was only now that I regretted bringing my laptop where I had removed the optical drive to save on wasted power and weight …

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Yep. The drivers still come on a CD. I had to do some furious digging before I found all the installers I needed and got the software downloaded over the hotel Wi-Fi.

20150212-1330-3524 20150212-1330-3525

The dongle itself was a medium sized unit, with no cap. Interestingly the printing on the bottom references DMB-T/H which means that it also supports the handheld-variant of the DTMB standard, although it is the old name for it.

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The tuner itself has a belling-lee PAL aerial socket, which was all good for me, because I had carried along F-to-PAL adapters in case I needed to plug it into a wall socket somewhere.

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As is the case for most tuners, they come standard with an IR remote and included batteries, not that anyone really uses them anyway. Not me.

It did come with a short USB extension (not pictured), which I thought was very thoughtful, as that solves the perpetual problem of port-hogging due to the tuner’s size

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It also came with a simple dipole antenna which is actually much smaller than it might appear – when folded, it would only be about 10cm tall. This was the antenna I used to receive most of my recordings, and I was surprised that it even got a signal at all due to the vast amount of local noise (e.g. LED light bulbs, multipath from buildings, water).

Teardown

Even before I began using it, the first instinct of mine was to take it apart and see what’s inside. And so I did, although, I didn’t have a good camera to take a picture of it at the time. So, when I got it home, I took it apart again …

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Well, would you look at that. RTL-SDR users would already see a breath of familiarity, as this is based on an RTL2836BU which is a cousin of our very familiar RTL2832U. This is paired up to a Fitipower FC0013 front-end (not our favourite) just underneath the IR receiver which is bent up and out of the way. Interestingly, the design of this tuner uses two crystals, both 27Mhz units. In most of the lower cost tuners, only one crystal is used with the RTL chip feeding out the clock to the front-end. Maybe there’s a limitation in the RTL2836BU or some stability requirements that couldn’t be met using that arrangement, hence the need for two crystals of the same frequency.

The marking on the PCB is HU772 Rev D. A quick search for the code suggests that the OEM may be MaxMedia.

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The underside doesn’t have much to offer, but it suggests the tuner was made week 38 of 2014. It seems there might be some power regulation circuitry on the underside, and possibly an EEPROM for the vendor information.

Watching and Collecting TV

Getting the tuner up and running wasn’t very difficult, with the Arcsoft TotalMedia viewer working out of the box. With some very careful antenna alignments, I was able to get all four multiplexes tuned and watchable but with some level of picture and audio break-up. This was actually quite variable, as the TVB and ATV SFN multiplex was nice and clear scoring above 90% decode quality, whereas the RTHK service and the TVB+ATV regular analog-replacement multiplex was variable, at times below 20% for quality and totally unwatchable.

If I was to catalogue and analyze the television in Hong Kong, using the TotalMedia viewer software was the wrong way to do it. It could only do one logical channel at a time, and it hid all the details about the tuning away from me. I had to dig a little deeper.

It was then, I realized, I still had my Twinhan TS Recorder application for BDA tuners with me. This little handy utility (which isn’t available anywhere else anymore) basically provides a crude front end to pass tuning commands to a DVB-T or DVB-S BDA compliant tuner and dump the resulting transport stream.

But hold on a sec, this tuner isn’t a DVB-T or DVB-S tuner …

You’d be right. But then I discovered something interesting – instead of making a whole new subset of a standard and cause the need for rewriting tuner applications to support DTMB, all DTMB tuners known to date exploit the DVB-T standard and present themselves as if they were a DVB-T tuner. Bingo! The capture application worked, as long as I plugged in the right frequency, and 8Mhz reception bandwidth.

Over a period of about a week, I managed to capture and carry home over 1.4Tb of TV data off the air for later viewing and analysis – everything including the null packets and TEI flagged reception errors. After all, I had no time to watch it live, and I didn’t have the time or tools to analyze it live either.

Channels on the Air and Proof of Reception

Although I was technically in the target reception area, the fact that I travelled all the way to Hong Kong to receive the signals makes it feel like a DXpedition to me. As a result, I wanted to collect something memorable and structured as proof of the reception. Of course, there was no time and it was unlikely they would issue QSL cards (it’s a bygone era), so I took to taking down the station watermark logos and recording their station promos and compiling them into a video. Not all clips were received without transmission error, but the majority were.

Stations in the TVB Network

Logo-TVB-Jade JadeLogo-TVB-HighDefJade High Definition JadeLogo-TVB-J2 J2
Logo-TVB-iNews iNewsLogo-TVB-Pearl Pearl

Stations in (or provided by) the ATV Network

Logo-ATV-Home Home
Logo-ATV-Asia Asia
Logo-ATV-World World
Logo-ATV-Classic Classic
Logo-ATV-CCTV1 CCTV-1
Logo-ATV-SZTV SZTV

Stations in (or provided by) the RTHK Network

Logo-RTHK31 RTHK 31
Logo-RTHK32 RTHK 32
Logo-RTHK33-CCTV9 RTHK 33 (CCTV-9)

More interesting is to watch the station introductions and promotional clips which I have compiled into two videos. The first video is for Hong Kong Free to Air stations which are locally sourced. The second video is for China TV provided via Hong Kong Free-to-Air stations. Enjoy.


Analyzing Hong Kong Free-to-Air DTMB Multiplexes

A nice thing about having transport stream captures is that you can take them home and analyze them at your leisure, using tools such as TSReader. Using this, you can read the system data from the transport stream and uncover streams which you didn’t realize were in the multiplex.

One of the first things I like to check out is the Network Information Table (NIT). The NIT usually provides information about the multiplex provider name, and modulation parameters. Unfortunately, due to the non-DVB nature of the system, the modulation parameters shown were completely wrong.

  • 402Mhz – Network name [blank], Network ID 32765
  • 586Mhz – Network name SFN_TVB, Network ID 32766
  • 602Mhz – Network name aTV-SFN, Network ID 32767
  • 802Mhz – Network name [blank], Network ID 32764

The frequencies used span the whole band from lower UHF to upper UHF, which seems like a rather strange arrangement especially if they want to refarm blocks of spectrum for resale. I do like the systematic nature of the Network ID allocations.

It was interesting to catalog the PID numbers, LCN numbers, encodings and bitrates of each channel in each multiplex. Again, the PIDs (in decimal) seem to be allocated almost systematically based on the LCN, rather than in the usual ad-hoc fashion. The results are summarized in the below table.

Multiplex-Stream-Summary

From the table, we can see that all channels except those owned by ATV are already running in 1080i. There are no MPEG-2 services on the air, and no MPEG-2 Audio anywhere to be seen. ATV broadcasts all their channels in standard definition except Asia, but they’re all in H.264 as well. Asia and High Definition Jade also provide 5.1 audio, whereas Jade and High Definition Jade provide triple-language subtitle streams. It’s important to note that Jade and High Definition Jade are not simulcast stations and often have different content despite the similarities in name. Jade itself is in High Definition.

On the 586Mhz channel, it seems there is a special DSM-CC stream available on PID1001. This stream carries 399kbit/s of data consistently, and its purpose seems to be related to the MHEG-5 TVB Interactive service. Unfortunately, never having used an MHEG-5 based service, I really wonder just how often used it might be.

To better understand the bitrate allocation, we need to know the encoding mode. Unfortunately, the data in the NIT wasn’t helpful, so lets take a look at the indicated air bitrate. TSReader reported a stream bitrate around 21.730 – 21.982Mbit/s when reading out from the file. This was true for all four multiplexes!

I had to look hard to find the DTMB modulation mode and bitrate tables, but I managed to find one courtesy of the Promax MO-270 DTMB Modulator User’s Manual. I have reproduced the relevant tables below:

4200-symb 4375-symb 4725-symb

Given the bitrate is around 21 to 22Mbit/s, the most likely candidate modes are:

  • 4200 symbols, 16QAM, 4/5 FEC
  • 4725 symbols, 64QAM, 3/5 FEC

Either mode results in the same net bitrate of 21.658Mbit/s, which we will use as the total stream bitrate. I’ve tallied up the streams and their bitrates, as well as that of the null packets – whatever remains belongs to the overheads of sending the PMT, NIT, SDT, and subtitles. The channel utilization is slightly below-optimal on the first transponder, but is pretty packed for everything else.

Mux-Bitrate-Distribution

Compared to DVB-T, their multiplex bitrates are actually lower than the rates most commercial channels run in Australia (see below).

Sydney Freeview Channel Modulation SummaryDifferences to Australian Television

Hong Kong TV does have quite a few notable differences to that in Australia. That includes:

  • The use of DTMB modulation schemes which are superior for signal robustness, and 8Mhz channel width instead of 7Mhz in Australia due to bandplan differences.
  • The widespread use of H.264 encoding, despite what the Wikipedia article states, MPEG-2 is no longer on the air. Likewise, 720p is not on the air either, and 576i is only restricted to ATV properties.
  • 1080i almost everywhere, and bitrates that are quite reasonable, exceeding 10Mbit/s in some cases (Youtube 1080p only averages 4Mbit/s). Using the H.264 encoding produces much better pictures for the same bitrate, compared to MPEG-2.
  • A lot of public service announcement ads – this includes anything from checking your mobile contract before you sign, being aware of high roaming mobile phone charges, to escalator safety, to keeping out of the way of emergency services when you drive, to competition watchdog messages, job scam alerts, election warnings, to blood transfusions, computer literacy, bird flu and AIDS warnings. They generally are pretty informative sometimes a little on the overdone dramatic/cute side, kind of like an infomercial. It’s not all about commercial ads!
  • They have sponsored time-check announcements of the time.
  • The watermark is on the top right hand corner, instead of the bottom right hand corner.
  • They make extensive use of multiple-audio technology to deliver programs in Cantonese and Manderin, or Cantonese and English simultaneously, and some have triple-subtitles as well, and not of the nasty blocky “teletext” type either! They look DVD quality.
  • The stations don’t always produce 24/7 programming. RTHK32 is mostly scrolling text most days, and RTHK31 is the same bar 3-6 hours a day of programs. TVB stations tend to repeat every popular program once to give viewers a chance to catch it in the evening if they missed out in the morning.
  • Syndication of content – stations seem to be happy to get content from elsewhere and air it on their channels. ATV, for example, carries CCTV-1 and SZTV from China, and RTHK carries CCTV-9 from China. ATV has ATV World, and TVB has Pearl which are 24/7 English language stations in a Cantonese speaking world.
  • CCTV-1 and SZTV via ATV is presented inside coloured pillarboxes because they’re native 4:3 content. We just use black pillarboxes or a half-way crop.
  • They have lots of station promos compared to us.
  • They don’t seem to use the EPG feature, or if they do, they don’t send them as MPEG-TS EIT events.

However, I think the biggest difference lies in the state of free-to-air in Hong Kong. While free-to-air here is considered viable and sometimes turns out decent programming, many viewers in Hong Kong have completely turned off free-to-air, and their locally produced content has never rated very well. This has combined into a slow death-spiral, which was consuming ATV while I was there. That was why they were running so many “new life” campaign promos, as they had run out of money and were unable to pay their employees on time, leading to a walk-out. The future of ATV is under question, given that they have started advertising extremely highly discounted advertising packages which are unlikely to turn a real profit. It’s a sad situation, as ATV has been important in the history of Hong Kong television.

So maybe I shouldn’t be so jealous of their high quality picture …

Conclusion

I can definitely tick the box and say that I’ve received DTMB-standard digital TV broadcasts and I know I have equipment that works. Analyzing the Hong Kong TV broadcasts has shown me the superiority of the DTMB standard, and the high quality delivered by the 1080i (on most channels) in H.264. It’s a shame that our local TV looks nothing like the quality of Hong Kong’s. Unfortunately, a quality picture doesn’t save the free to air networks from declining ratings and viewership, and ATV’s future is currently under a shadow.

About lui_gough

I'm a bit of a nut for electronics, computing, photography, radio, satellite and other technical hobbies. Click for more about me!
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8 Responses to Trip to HK & CN 2014 – Part 3: Hong Kong Free-to-Air DTMB Analysis

  1. Pingback: Trip to HK & CN 2014 – Part 5: RTL-SDR, More Radio, AIS & Night Photos | Gough's Tech Zone

  2. Pingback: Project: Scanning Sydney’s DAB+ & FM Broadcasts | Gough's Tech Zone

  3. 李耀宗 says:

    Interesting reading on your HK TV analysis.

    Here are some comments:

    1. Actually every HK family tunes to the free-to-air TV, but quite some just ignore it (until the disappearance of ATV and appearance of ViuTV).

    2. Not many people is subscribing to Cable TV now, which is notorious of difficulties on unsubscribing it. (And there was a knife attack at Cable TV headquarters for unsubscribing matter, no joke!). Those subscribed to Cable TV in the past was mainly for watching overseas soccer match (e.g. England Soccer Premier League, which is not available on free-to-air TV).

    3. DTMB modulation mode is 4725 symbols, 3/5 code FEC, 64QAM. The broadcasting standard is available from http://www.coms-auth.hk/en/policies_regulations/cop_guidelines/broadcasting/television/index.html

    4. MHEG-5 is mostly useless, mainly broadcasting news and stock market.

    5. There is EPG in Hong Kong, and sticks to DVB standard.

    6. ATV stopped broadcasting at Apr 2 2016 00:00 HKT, and ViuTV starts broadcasting at the same time.

    • lui_gough says:

      Thanks for the update! It was definitely historic when ATV stopped broadcasting. I suppose next time I arrive in Hong Kong, I might see ViuTV on the air :).

      – Gough

  4. Joao Castanheira says:

    Hi! Many many thanks for this post. That’s something that I was really looking for. I live in Brazil and every place in the world that I go I make TV recordings. Could you kindly inform where the USB was bought? I’m going to HK early next year. My best regards

    • lui_gough says:

      Glad to hear that. Unfortunately, when I was in Hong Kong, it was quite difficult to find them even in 2014. I doubt they would be easy to come by – your best bet is to visit the computing super-centers such as Golden Computer Center in Sham Shui Po or the Computer Center in Wan Chai. The unit I have was purchased from Foresoon Computer in their Wan Chai branch, but I’m not sure they stock such units anymore due to the unpopularity of “free to air” TV as compared to cable in Hong Kong.

      – Gough

  5. Marcus says:

    Great article, if someone could please, please recommend where USB DTMB stick can be purchased on-line and it is supported in Ubuntu.

    Thank you so much.

  6. mamborambo says:

    Great to read about digital tv technology from someone who is a knowlegeable insider! I live in Singapore which is transitioning to all digital DVB-T2 by end December 2018, so furiously upgrading all my equipment and trying to understand how to connect DVB-T2 to my homebrew boxes. The process of learning about terms and standards show me how much snake oil were being peddled — everyone kind of know but don’t really understand what is happening… story for another time.

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