Wednesday, July 5, 2017

Wiistar 5.1 Audio Decoder Teardown

For some experiments that require hardware decoding of Dolby Digital I've acquired a cheap Chinese 5.1 decoder on Amazon -- it costs just $24 so there was not much hesitation while buying it.


The good news is that it's indeed a proper Dolby Digital (AC3) decoder, which also supports upmixing of stereo channels into 5.1 (probably using Dolby Prologic). The bad news is that the quality of the audio output is... consistent with the price of the device.

I've found a post by Alexander Thomas describing previous versions of this device. Compared to what Alexander had observed, the hardware I've bought seems to be somewhat newer:
  1. Instead of CS4985 decoder chip it uses an unidentified DSP chip of a square form.
  2. There is no filtering of the output signals or any "bass management" (sinking of low frequencies from the main channels into the subwoofer's channel).
  3. The unit is powered from a 5 V source instead of 9 V.
  4. The unit provides a 5 V USB power outlet.

There are still some similarities though:
  1. LFE channel lacks +10 dB boost expected by the DD spec.
  2. The board's ground is not connected to the case.

Hardware Teardown

Now let's take our screwdriver and see what's inside the box. This is how the board looks like:


Most of the components are mounted on the top side. Some of the major components can be identified:
  • [1] 4558D is a stereo opamp, this make is by Japan Radio Company (JRC);
  • [4] ES7144LV is a stereo DAC -- the board employs three DAC / opamp pairs;
  • [7] 25L6405D chip is flash memory;
  • [6] NXP 74HC04D is hex inverter chip;
  • [2] AMS1117 is power regulator.
There are two mystery chips:
  • [5] the big one labelled VA669 -- I suppose that's the decoder DSP, having that there are traces coming from it to the DACs, but the actual make and model of the chip are unknown;
  • [3] the one labelled "78345 / 8S003F3P6 / PHL 636 Y" -- judging by its position on the board, it could be a microcontroller handling input selection and "5.1 / 2.1" switches.
And this is the bottom view:


One interesting thing to note is that the labels and holes suggest that this board can be equipped with RCA output jacks per channel, as an alternative to three 3.5" stereo jacks and the 5 V USB outlet. This suggestion is confirmed in the manual:


Measurements

I was wondering whether this device can be used in any serious setup, and for that I've hooked this device up to the inputs of MOTU UltraLite AVB audio interface.

I needed a test sound file that is AC3-encoded and contains measurement sweeps in all 6 channels. For that purpose, I took the measurement sweep file generated by FuzzMeasure, and used Audacity in order to create a 6-channel file with a sweep in each channel:


Note that ffmpeg library which is used to encode AC3 applies a lowpass filter to the LFE (4th) channel. This will prevent us from seeing the full performance of the LFE channel on the device.

Using a TOSLINK cable I hooked up the device to MacMini's optical output, played back the encoded file, and recorded the decoded analog output using MOTU.

The first thing I discovered was that the surround channels are swapped. That is, they use a reverse of the standard TRS stereo channels mapping where the left channel is on the "tip" contact plate, and the right channel is on the "ring". Instead, the left surround is on the "ring", and the right surround is on the "tip". Perhaps, this was done on purpose to undo the reversal of "left" and "right" if one sets the surround speakers facing him, and then turns around :)

The next discovery was quite a bad shape of the output waves. As one can see, the sine wave is severely clipped at bottom half-waves. This is how the source -3 dBFS sine wave has been rendered:


Input sine wave with smaller amplitude (-6 dBFS) is clipped a bit less:


This is very unfortunate, and is probably caused by a bad design of the output stage. Looks like using the 4558 opamp wasn't the best choice in the first place, and the designers of this board seriously hindered its performance by failing to drive it correctly.

After looking at these horrible output sinewaves, I wasn't expecting a good frequency response, and indeed it's quite bad. Below are the plots for the left channel from a -3 dBFS input signal (blue), and for -6 dBFS input (orange), no smoothing:
The measurements for the remaining channels are the same as for the left -- at least this device is consistent for all channels. Below is left channel (blue) vs. LFE channel (yellow):
This plot confirms that the LFE channel has the same output level as other channels, lacking the required +10 dB boost.

It's very funny to look into the "Technical Data" section of the manual for this device, stating "Frequency Response: (20 Hz ~ 20 KHz) +/- 0.5db":

The authors tactfully omit the level of the input signals used in this measurement (if it actually was performed) -- probably the level wasn't too high.

Conclusion

Looks like this family of devices can't be used in any serious setup. It will be interesting though to try to reverse engineer the electrical design of this board, and fix obvious flaws.

14 comments:

  1. Hi dude, I have the same gadget. Did you have any problems with sound delay?

    ReplyDelete
    Replies
    1. Have you solved problem of sound latency?

      Delete
  2. Hi Jorge, I didn't use it together with video, so I couldn't detect whether there is a delay.

    ReplyDelete
  3. Wayne, for some reason I can't see you comment on site, only on the RSS feed.

    > Did you give up on this? I would like to create a better version.

    I didn't have much pressure so far to try to fix this unit.

    In fact, if you want to have a surround decoder with similar capabilities, but not being crap, and for a reasonable price, look for old Oppo Blu-Ray players. They perform flawlessly.

    ReplyDelete
  4. Hi,
    I'm trying to convert this decoder into something useful.
    Your measurements are very helpful.
    My advice:
    Replace 9.1k Ohm resistors (near opamps) with a higher value 18k or 22k, it will reduce the gain and solve the clipping problem.
    Replace DACs with CS4344 they fit without modification.
    Replace opamps with any 'rail to rail' matching 5V power supply.

    ReplyDelete
    Replies
    1. Thanks a lot! I will try that.

      Delete
    2. Did you can solve "latency" problem?

      Delete
  5. Михаил, здравствуйте. (надеюсь с языком я угадал) =)
    Купил в прошлом году такое устройство. И до сих пор мучался пока не разобрал и не стал гуглить. В результате увидел ваш пост. Устройство по начинке 1 в 1 и имеет теже самые проблемы. А именно перепутанные правый и левый фронты. Самое главное как написал один из читателей выше это отставание звука. При проигрывании аудио это возможно не проблема но при просмотре кино просто ужасно. Начал читать отзывы с амазона по вашей ссылке оказалось у всех эти проблемы. Отсюда возникло несколько вопросов....
    1. Вы модифицировали устройство? (человек выше написал как)
    2. Как решить проблему со сменой канала?
    3. Вы уже проверяли отставание звука? (если да то как решили проблему?)
    4. Не удалось ли идентифицировать чип?
    5. Не нашлось ли аналогичного устройства по похожей цене с нормальной работой?
    6. Есть ли что-то подобное на 7.1 но не через HDMI?

    ReplyDelete
  6. Вопрос по поводу отставания звука не только к вам.

    ReplyDelete
  7. Здравствуйте, Сергей! От использования данного устройства я отказался и пока к нему не возвращался. Вместо него использую Blu-ray плейер Oppo 103. Поскольку это уже устаревшая модель (нет поддержки 4K Bluray дисков), то можно найти б/у недорого. У него есть полная поддержка Dolby Digital Plus (7.1) и аналогичных кодеков от DTS, нормальный бас-менеджмент и цифровая задержка каналов для компенсации разных расстояний до колонок. Ну и качество не сравнить с китайской поделкой от Wiistar.

    Однако внешние входы у 103-го только HDMI. По TOSLink передать DD+ нельзя в принципе в силу ограниченной полосы пропускания данного интерфейса. Если вам нужно конвертировать аудио между HDMI и оптикой, есть чудесный девайс от Monoprice: https://www.monoprice.com/product?p_id=5369 Работает тоже безо всяких нареканий.

    ReplyDelete
    Replies
    1. Спасибо за ответ. Как я понял из своих поисков это копия, копии, копи... И правильный варинт только с правильным чипом (как в ссылке на статью доктора Лекса) я кстати пообщался с ним. и в принципе он подтвердил что на его устройстве задержек небыло. У меня не открылась ваша ссылка даже через анонимайзер. Ну это ладно. Главное что меня удивило это цена OPPO 103 не знаю что вы имели ввиду под невысокой ценой...но я пока ивжу не менее 1000 долларов.

      Delete
    2. Дело в том что через HDMI ARC тоже не передается 7.1 т.к. по сути это тот самый SPDIF

      Delete
    3. хотя вот тут https://en.wikipedia.org/wiki/S/PDIF
      говорится что и 7.1 поддерживает...я понимаю что википедия не панацея. а где бы точнее почитать. информация все противоречивее.

      Delete
    4. Спасибо за комментарии! Уточняю:

      На штатовском eBay BDP-103 б/у можно купить от $250.

      Монопрайсовский девайс называется "DVI Video + Digital Coaxial and Digital Optical Audio to HDMI Converter"

      Да, есть кодек DTS-ES который кодирует 6.1 и может передаваться через SPDIF. Возможно в статье из википедии имеют в виду его. Но более новые (хотя уже устаревшие) кодеки для 7.1: Dolby Digital Plus и DTS HD не пролезают.

      Delete