TRAXMOD

Developer's Area (schematics, firmware, etc.) FAQ Support Forum Project Background YouTube Demonstration Resistor/PWM Hybrid DAC SD Cache Methodology SD Theoretical Limits SD Card Interface MOD Effects BBMICRO Protoboard LPC2103 ARM7TDMI-S

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TRAXMOD is a small digital audio player using the LPC2103 ARM7TDMI-S high speed microcontroller chip (prior versions were based on Microchip dsPIC DSPs). Currently, TRAXMOD can play raw wave audio and 4 channel MOD files streaming off of a FAT formatted MMC/SD Card at 44100Hz, 16 bits, stereo. The hardware for TRAXMOD/ARM can be built using a BBMICRO Protoboard and a few additional low cost components.

For more information about TRAXMOD, please read the FAQ. To see the current player in action, check out our YouTube demonstration video.

News

07-04-2008

Got it down to only 6 air-wires now. Transparency of the panelized PCB is printed out and looks pretty good (no smudges or hairline grain ink bridges). Tomorrow morning I'll try to clean up some of the dust in my work area to avoid possible contamination of the transparency image. Then it'll be time for lights out, UV exposure, and etching of a hopefully good circuit board. Exciting!

Been working like crazy on designing a TRAXMOD-PIC32 PCB. It's such a small, simple feature, but I totally love the PIC32's programmable internal pull-up resistor pins (LPC2103 requires external pull-ups). This has made it so much easier to route traces for each button, allowing me to do almost everything on a single layer PCB design, reducing component count and board space. I think I've got the whole thing routed now, with only 7 air-wires. Busy panelizing it now, and will hopefully spend the coming Fourth of July weekend etching the boards.

Submitted an order at Mouser for some more parts I'm going to need for this PCB, plus a few extra parts to play with for future projects. Damage done: $155.

Firmware v0.6.8 posted in the Developer's Area. This release makes some improvements to the filesystem code so that files with more than 7 characters will be picked up by the player. Previously, it was impossible due to some crappy design in the EFSL interfaces. After downloading a directory full of MOD files from Amiga Music Preservation, I added a feature where the player will now also play MOD files with the MOD extension in the front, the way Amiga used to store them. This makes it much easier to simply unzip, and then dump those Amiga MODs straight to your SD Card, without having to go through renaming each file.

While I was messing with the filesystem code, I ended up adding a feature to save current play list location and volume settings to a TRAXMOD.CFG file on the SD Card. Now when you restart the player, it will always start right up playing the file you were last listening to with the volume at your desired level. Very nice.

In other news, the PIC32 player parts are starting to come together. I now have the PIC32 Starter Kit in my hands and a bare AV32 breakout board. While I have not put a soldering iron to anything yet, I have started playing with the free C32 compiler and managed to blink an LED without any library code linked in. While I haven't done any tests to prove it, I believe this means that my simple C firmware is completely free of the 64KB FLASH memory limit that the free compiler libraries would normally impose. That was easier than I thought it might be, dunno why people would even bother complaining about it. Having full optimizations is far better than a stupid library that might bloat up your firmware with unnecessary code anyway.

The only PIC32 snag I noticed while reading this weekend was that the 64 pin version of the PIC32 does not have any pins to input an external clock to drive Timer 2 or 3, which in turn drives PWM output compare pins. In the LPC2103 based player, we currently use the DAC's bit clock as an external timer clock source so that we can precisely position our PWM output signal to activate the SPI slave select signal during the upper 16 bits of the DAC's 32 bit word transfers. Not having external timer clock inputs on the 64 pin PIC32 means we're going to need to take another approach. Because the PIC32's SPI port can output a full 32 bit word and has a buffer register (the LPC2103 SPI port has neither), we might be able to do without a slave select signal entirely by simply keeping the SPI transfering words continuously, back-to-back. I briefly tried doing this on the LPC2103 before, but it didn't work. Most likely due to the lack of a buffer on the LPC2103 and because its SPI can only do up to 16 bit word transfers.

My PIC32MX320F128 and PIC32MX320F064 low memory TQFP-64 samples have actually shipped now. Should be in my hands sometime this week. Also, the TPS61025 samples are in my hands right now, so we're good to go there too.

Made my first YouTube video. You can watch it here: TRAXMOD-ARM v0.6.7 MOD Player Demonstration. Unfortunately, the audio compression makes the high frequency chimes in the beginning raw audio demonstration sound atrocious, but you can be assured that the actual DAC output of these audio files is very good in reality. There is also a little buzzing noise apparent at the beginning of the video. I'm not quite sure why it's happening, but it goes away when I unplug the USB serial port dongle from the player, so it doesn't seem like too big a deal. However, I needed the serial port to see on the terminal when my button pushes actually registered for the video (I couldn't listen to the audio while the video was being recorded).

Firmware 0.6.7 has been posted under the traxmod-arm-0.6 folder in the Developer's Area. This version has smoother volume control while playing MOD files, and no longer locks up on any of my MOD files that I'm currently testing. This version sounds quite good, at least on my Dane-Elec 2GB SD Card, which seems to provide the lowest seek latencies of all my SD Cards. I can hear some occasional minor MOD sound glitches on my other SD Cards.

I just took some quick measurements with my multimeter for power consumption. This version seems to use about 70mA total while playing raw audio files, and about 80mA total while playing MOD files (not really any different than prior versions). The raw audio player could probably be optimized for power consumption by re-programming the PLL to operate at 50MHz instead of 70MHz, but we haven't bothered to implement such a thing yet. Not sure what could be done to improve the MOD player power consumption because we really need the full 70MHz at the moment to keep the SD Card clocked up. Maybe if there was more on board RAM for a larger output audio buffer, the mixer could sleep a little bit. Once we get the PIC32 going, we'll finally have that extra on board RAM (hopefully).

Ordered some TPS6102x samples from TI. Unfortunately, their order form is insistent on requiring a phone number. I hate getting follow up phone calls from salesmen. If I'm ordering samples of something, obviously I've already researched using their product to begin with, so why do I need a salesman wasting my phone minutes and time that I could be spending on making a final product? I put in bogus number 000-000-0001 to get past the validation code. If they cancel my order because of it, I'll either have to think about finding a different part or just buy the part from Mouser/Digikey next time I place an order there ($3.15 is worth avoiding sales phone calls).

Anyway, back to the TPS6102x parts. These are DC/DC boost converters that seem very flexible and efficient. They are a little more expensive, but might be worth it in the end. They can operate from a minimum input voltage of 0.9VDC, which means you could power the player from only one cell (but efficiency would be lower). They also have an alternate LDO mode of operation that automatically kicks in when the input voltage is above the desired output voltage, letting you use 3 or possibly more cells without fear of killing your circuit (but with even lower efficiency when operating in LDO mode). I'm thinking of 2 cell operation for maximum efficiency, although my cheap EMPREX MP3 Player from Frys Electronics is rather nicely compact with only one AAA cell.

The alternative part I was looking at this weekend was the Semtech SC4501. This DC/DC boost converter doesn't have the fancy LDO feature and requires 1.4VDC minimum input to operate. That rules out using anything but 2 cells (which is fine with me). Unfortunately, efficiency falls when the load current goes below 100mA, so this is not my #1 choice. It is much cheaper than the TI part though.

Ordered a $55 PIC32 Starter Kit Breakout Board that I ran across here: eflightworks.net. All of my TQFP-64 PIC32 samples are apparently back ordered forever, but at least I'll be able to start porting software over to the 100 pin Starter Kit in the meantime. Plus I won't have to waste any time designing a development board of my own.

Not too much to report. This weekend was spent on attending Desert Code Camp. Mostly Windows software development, but there was some interest in embedded Ethernet (unfortunately not much new content for me though). Maybe if I get a chance, I might try doing my own embedded systems presentation at a future Desert Code Camp.

PIC32MX340F256H samples still haven't shipped, and their estimated in stock date has been bumped forward another month. As a result, I put a new sample order in today for the lower memory versions of this chip in the 64 pin package, hoping they will ship. Being in the same package, I can probably use these smaller memory versions to produce a PCB and some initial firmware until the full version arrives.

While I was at it, I also requested the DSPIC33FJ128xx802-I/SP chips. Last time I used a dsPIC, they were only 30MHz, had tiny memory compared to LPC2103, needed 5V, and didn't have any DMA features. These newer dsPIC's have DMA support, 16KB of RAM, 128KB FLASH, and 40MHz operation at 3.3V. Still not as sexy as the PIC32, but since they come in a handy DIP package straight up, they still might be useful for beginning work while a TQFP 64 pin breakout board is being made.

It's been a very nice Memorial Day weekend for the TRAXMOD project. Schematics and firmware for version 0.6.6 has been posted in the Developer's Area. This version features the following:
• Externally clocked DAC and simplified word clock syncronization code.
• Many bugs fixed, far fewer audio glitches in MOD playback.
• Full 8 byte SSP FIFO usage in the SD Card reading interrupt service routines to reduce task switching overhead.
• Converted SD Card interrupt service routines to pure assembly for additional speed and code space compactness.
• Support for SD Card multi-block reads so that we can fast forward past block mis-aligned data (ISR only saves actual sample data into the 512 RAM buffer, no junk data wasting buffer space and no need to read two whole blocks).
• Music event frames no longer block the mixer code from continuing work on all channels. Music events are double buffered now, so that music processing can occur one row ahead of where the mixer is currently playing.

At this point, I'm thinking any glitches in the MOD player audio are no longer being caused by lack of data being ready in time from the SD Card. I think the remaining few glitches are from bugs in the code, and/or lack of volume ramping support in the mixer. It locks up on a few select MOD files, not sure why yet. A couple MOD files are able to play sample data that they should not be able to access. So, still plenty of work to do, but some MODs sound very close to perfect (at 44.1KHz/16-bit stereo, no less!).

Spent some time playing with different clock configurations. If I clock the LPC2103 at 40MHz or below, it doesn't communicate with the externally clocked DAC very well any more (I get crackly audio that gets worse at each lower clock speed). At 50MHz and above, communication with the DAC is okay.

If I try to clock the SSP communicating with the SD Card at 50MHz/2 = 25MHz, almost all but one of my SD cards stops working. Maybe the SD Card 25MHz specification limit means you really have to use some clock speed below 25MHz. Maybe the air wire signals on my bread board aren't good enough to support 25MHz communication? Anyway, I'm stuck running the LPC2103 at 70MHz and clocking the SD Card at 70MHz / 4 = 17.5MHz.

In other news, I've been trying to optimize the MOD player code like crazy. I now have the SSP interrupt handlers using the full 8 byte FIFO to reduce interrupt overhead. I also split the channel data structure into two separate structures so that the music event decoding can occur further in advance of the real time audio play back pointer. These two optimizations reduce the audio glitches a lot, but unfortunately, there are still some occasional problems on certain MOD files. Either something is still too slow or I may have some problems with my channel scheduling algorithms. Plus, it locks up sometimes at certain spots on certain MOD files now.

I was playing Terraforma by C.C.Catch on my TRAXMOD player when I thought it was sounding rather odd. Turned out my MOD player wasn't buggy, but the copies of terraforma.mod that I could download from AMP, Mod Archive, and ftp.ModLand.com all had bit rot in this file. At some point, two bits in the sample length fields of the MOD file had been flipped, making half of the sample data misaligned with what the sample header information was saying. After analyzing the problem for a while, I found the two bits that were wrong, whipped out a hex editor, and fixed the MOD file. You can download the fixed version here: Terraforma.mod (fixed).

In other news, I'm now clocking my DAC with an external 11.2896MHz crystal rather than using a timer output of the LPC2103. High frequency audio seems a little more sharp now than before. This is probably the maximum audio quality possible for 44.1KHz audio using this DAC. It also frees the LPC2103 to run at it's own clock rate, which we could use to drive the SD Card closer to it's maximum of 25MHz, or to run it at a speed that will allow maximum FLASH bootloader serial port baud rate.

I'm starting to hit the 32KB FLASH memory limit of the LPC2103. Still need to add lots more code to the player to support more module formats and to further optimize playback to reduce audio glitches during MOD playing. Looks like it's getting close to time to start work on switching to a larger and slightly more expensive microcontroller. I've started work on an EAGLE part library for the new PIC32MX340F256H microcontroller. With the added memory and DMA features, I'm hoping we'll be able to play multichannel module files beyond 4 channel MODs after the switch.

Booya! Figured out where all the audio crappiness is coming from. It's not an off by one bug at all. Turns out it's caused by SD Card seek time stalls of the audio mixer. I changed the code so that it will attempt to load the RAM buffers with block aligned data, even if some of that data isn't going to be used by the mixer, and that improved the audio quite a bit. I still get glitches on certain particularly poorly aligned samples, but it's way better. Sweet!

I've still got to implement my idea for improving the channel scheduler to look for block aligned channels when the mixer is starving. But it's late and I have to go to my day job tomorrow morning, so that idea will have to wait for another time.

Firmware 0.6.4 posted in the Developer's Area. It actually sounds rather decent during portions of certain MOD files now.

Firmware 0.6.3 posted in the Developer's Area. This release uses the streaming MOD player mixer, so you can play any size MOD file without worrying about fitting within the LPC2103's tiny 8KB of RAM. Only 4 channel, 31 instrument MOD files are supported at the moment. It plays MODs directly off a FAT formatted SD Card (yay!), but it will play incorrectly if the MOD file itself is fragmented by the FAT file system on the card.

There are still some very annoying bugs with this firmware which make the audio output very poor compared to the RAM only MOD player firmware in earlier releases. I did at least get it to the point were it doesn't outright lock up in the middle of most MOD files though. I'm probably off by one somewhere, but it's been impossible trying to find it.

At first, I thought maybe the poor audio was from lack of speed in the new streaming code, but lowering the DAC output sampling rate to 22050Hz did not provide any relief. Must still be bugs somewhere in the implementation.

Identifying and fixing bugs in the streaming MOD player branch of the firmware source code. It still has a lot of audio glitches, but the output is recognizable. I think I might have some firmware stable enough to post at the end of this weekend.

Been way too busy at my day job, doing taxes, battling bees in the backyard, and helping my brother move, but I managed to sneak in some tiny pieces of time on the MOD player project during the past few weeks (mostly during dinner time). I've re-written the ARM mixer code for SD Card sample streaming, which will allow playback of any sized MOD file because we don't try to load all of the samples data into RAM before beginning playback. It seems to be producing somewhat recognizable audio on a two channel MOD, but there needs to be a lot more optimizations to the code to get it going fast enough to keep up with the output audio rate. Right now, it has some very heavy reverb distortion because it can't keep up (at least, I hope that is what is going wrong).

I've got a lot of optimization ideas, some are quite involved though (hardware changes, plus new components that I'll need to order before I can do it). For example, reading data from the SPI port in master mode requires twice the number of instructions than should really be required because we have to pump the SPI data register with dummy data to get the SPI peripheral to clock in each SD Card byte. That's just plain wasteful of instruction cycles. Plus, our SPI master mode clock runs at a slower speed than the maximum 25MHz accepted by SD Cards because we have to use a multiple of the LPC2103's clock rate (I think we're currently maxed out at 16.9344MHz). If we had an external free running clock, we could run it at the full 25MHz, throw the SPI port into slave mode, and then we would not need to pump the SPI data register with dummy data to receive data from the SD Card. Unfortunately, I do not have parts necessary to create a good 25MHz clock source externally on hand right now. Plus, this change would necessitate delving into the EFSL FAT library source code to teach it how to access the SD Card while the SPI port is in slave mode. Lots of work to be done.

Meanwhile, I think I've squashed the bug that was causing some MODs to still have samples that don't play correctly in version 0.6.1. In order to print out sample names, I was writing a zero to the last byte of the sample name data, or so I thought. Instead, I was off by one and ended up zeroing the most significant byte of the sample length variable in memory. That caused larger samples to get truncated in memory. The version 0.6.2 firmware snapshot available in the Developer's Area now fixes that little bug. All of my MODs that are small enough to fit into RAM now play much better with this version. Aside from missing effects like arpeggio, I'd dare say they sound close to perfect.

New firmware snapshot posted in the Developer's Area. Version 0.6.1 fixes several bugs in the MOD player firmware. MOD playback now plays at the right frequency, the audio mixer now avoids a sign extension problem through the use of inline assembly code, but some MODs still have samples that don't play correctly to me. Still, it's much better than before.

New schematic and firmware snapshot posted in the Developer's Area under the "traxmod-arm-0.6" folder.

The raw audio player using the PCM1770 DAC is working splendidly now. I wrote some DAC word clock synchronization code last night/this morning and I haven't been able to get the player to start up unsynchronized ever since. Sound quality is excellent, volume control works nicely, the code is a bit simpler than the PWM/Resistor DAC code, and the schematic is simpler too. Go to the Developer's Area and check it out!

Well, after a quick sleep, I got right back to work on the project. I've now gotten it to work without the dsPIC30F3012! There is still some trouble with getting synchronization between the DAC's word clock (LRCK) and my LPC2103's SPI slave select signals, but when it works, it works nicely.

Stayed up all night long and got the PCM1770 DAC running in master mode! I'm still using the dsPIC30F3012 to send the control port commands that tell the PCM1770 what volume to use and to go into master mode. It's got some initial synchronization issues I'm working on too. I'm thinking I might try hooking the control port up to the SSP module shared with the SD Card next to get rid of the dsPIC.

All my efforts to bit-bang the PCM1770's control port so far have failed. Today I decided to try using the SPI port on my dsPIC30F3012 to send volume control commands to the PCM1770. Worked straight away, excellent. Now I can listen to the DAC directly with my Koss Porta Pro head phones. Great! Haven't tried enabling master mode yet, but should be easy. I'll still need to eventually get some software written to properly bit bang just like the SPI port, otherwise I'll have to share pins with the SD Card's SSP module, which I'm hoping to avoid.

Sweet! I just got some audio from the PCM1770 DAC using a dsPIC30F3012 to provide LRCK, BCK, and SSEL clock signals to the PCM1770 and LPC2103 at the appropriate times. Still can't seem to bit bang the PCM1770's control port lines correctly though. Makes it difficult to use the digital volume control features, ugh... The sound quality when piped through my stereo's volume control sounds good. Hopefully we'll be able to get this control port fixed up quickly and then I'll listen to the DAC directly with my headphones.

Spent the whole weekend trying to get the PCM1770 DAC up. Unfortunately, I can not get the thing to go into master mode. I'm providing it a 11.2896MHz input clock (256 x 44100) from one of the LPC2103's timer match output pins. I pull PD# low for a while to reset the DAC, then bring it high to release it from reset. I try writing 5 to register 3 of the DAC, which ought to switch it to master mode, but nothing comes out of the LRCK and BCK pins of the DAC. Very frustrating.

My next plan of attack is to temporarily wire up one of my dsPIC30F3012's to generate all of the clock signals I need to use the DAC in slave mode. If I could get it outputting audio in slave mode, at least I'd know my DAC is working. Then I could go back to trying to get the DAC to switch to master mode in order to avoid needing the dsPIC.

It took a long time, but the 16pin SSOP to DIP PCB's from Forest City PCB finally arrived today! They look very nice quality, better than I was expecting. I don't have time to solder things up right now, but I placed the chip on top to take a look and it looks like it lines up perfectly. Hope I get some free time to play with it soon.

Well, got tired of trying to put together a panel of PCBs just to get a SSOP-16 to DIP converter to start playing with the PCM1770 DAC samples. Ended up ordering three 16 pin SSOP to DIP PCBs from Forest City PCB for $1.49 each plus shipping. I've never ordered from them or heard of them before, so hopefully they're legit. It would be cheaper to just etch them on a single layer paper phenolic board (PP101 is $2.95 at web-tronics.com), but these days, the couple of dollars saved just doesn't seem worth it compared to the time involved in etching a PCB by hand.

Placed an order at Digikey for some crystals that operate at 16.9344MHz and 11.2896MHz. These frequencies are exact multiples of 44100 Hz, which is the standard audio CD sampling frequency. You would think that getting crystals for audio would be easy, but it wasn't. Few manufacturers make them, and they are often out of stock at Mouser/Digikey, unless you want to pay extra for an overpriced part.

When using the PWM-DAC, it wasn't too hard to play 44100 Hz audio without a perfect clock multiple because we could control in software how fast we switch to the next sample. But using the PCM1770 DAC requires us to provide an input master clock that is 256 times the playback frequency. If we have the LPC2103 clocked at 70MHz and divide it down to provide the PCM1770's master input clock, then the closest playback frequency will end up being 39KHz or 45.5KHz. That's pretty far off and would be noticeable.

Still haven't made any PCBs yet. Thinking of doing it by hand this time instead of ordering it manufactured. Not making a final board, just another prototyping board for playing with the DAC and the LPC2103 timer match schemes. Hopefully will have enough time to get something done.

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