5 ms·
Not the same kind of noise, but this reminds me of a college project from college when I was studying electrical engineering. We were building an MP3 player us
by hermitdev 7y ago
Not the same kind of noise, but this reminds me of a college project from college when I was studying electrical engineering.
We were building an MP3 player using an Atmel AVR dev board with off board circuitry for the MP3 decoding and DAC for output. We couldnt get the MP3 decoder working, despite us appearing to be sending all the right signals/data. Everything looked fine in the logic analyzer. We werent running at super high frequencies, either. Maybe just a few MHz.
We didnt find the problem until someone bumped the frequency scaling, so we were looking at the signal in much higher frequencies. Thats when we saw all of the noise in the signal. We had all sorts of jitter from signal bounce. Our external board was connected to the dev board via an 18" ribbon cable and we had forgotten to properly terminate the bus.
It was really embarassing as we had just taken travelling waves the semester before and should have thought about it. But, hey, we were students, and undergrads at that, so definitely a learning experience and reinforced a class we were required to take and thought was useless at the time. Lesson learned.
For anyone curious why we would build a hardware MP3 player, this was back when the only HW portable players were the early Diamond Rio's (think they only had a 32MB and 64MB model available at the time).
Edit: typo MGHz->MHz
- C1sc0cat 7y agoI still have my first MP3 player a 64mb Diamond Rio in a draw at home, I wonder if it still works.
- tyingq 7y agoI remember when the hard drive mp3 players came out, like the first iPod. As a sysadmin at the time, I found it amusing that people were wearing these while running, etc. Worked out better than I had imagined. Did they use "special" hard drives with some sort of unique head crash protection?
- jzwinck 7y agoThey did not use special drives, usually 1.8" drives identical to what was in the smallest portable hard drives. I remember because I fixed a friend's dropped iPod by buying one of those enclosed hard drives and transplanting it.
- eigenvalue 7y agoI think they did use some kind of inertia sensor that would detect when the device was falling and spin down the drive to reduce the chance of it breaking.
- lonelappde 7y agoThe used a SSD buffer of 16-64MB so that the hard drive spun up only once every few minutes of playback. But navigation during movement would still be risky.
- jacobush 7y agoThat doesn't sound right - that would have been a significant extra cost.
- tyingq 7y agoI don't see any notion of an SSD in the old iPods, but they did seem to have 32MB of RAM. I assume some was left for use as a buffer.
- serf 7y agoEach iPod also has 32 MB of RAM, although the 60GB and 80GB fifth generation, and the sixth-generation models have 64 MB. A portion of the RAM is used to hold the iPod OS loaded from firmware, but the majority of it serves to cache songs from the storage medium. For example, an iPod could spin its hard disk up once and copy approximately 30 MB of upcoming songs into RAM, thus saving power by not requiring the drive to spin up for each song. wiki entry on iPod Classic. : https://en.wikipedia.org/wiki/IPod_Classic https://en.wikipedia.org/wiki/IPod_Classic
- fzzzy 7y agoThat's exactly how it worked.
- lonelappde 7y agoAs others said, I misremembered RAM as SSD. Mandela effect!
- hazeii 7y agoI have a bunch of them, and they do still work (if you're using them under linux, you're running our code [0]). [0] https://www.zdnet.com/article/hacked-rio-to-pose-legal-problems-5000101466/ https://www.zdnet.com/article/hacked-rio-to-pose-legal-probl...
- jzwinck 7y agoNot only did the Diamond Rio 500 top out at 64 MB of internal storage, it had a rather famous hardware bug where it would lose capacity (permanently) if it lost power during a file transfer. Mine lost about 8 MB. Others lost even more. The other cool thing about it is that it has a USB mini plug but does not use a standard USB mini cable. If you use a standard cable it can destroy the player. Oh, and it supported a flash memory card for more storage...maximum 32 MB. Oh well.
- m-p-3 7y agoIf only Opus (codec) was a thing at that time, could have put some music at a relatively recent quality without using a lot of storage.
- derefr 7y agoWhich is kind of like saying “if only we had H.265, VCDs could have been high-quality.” We certainly could have invented the codec back then (maybe, if we had had a few more genius information-theory mathematicians born the years before), but could we have implemented it in silicon with 1990s tech, with an affordable power budget (e.g. two AA batteries)?
- pjc50 7y agoSpoiler: no, there wasn't enough CPU power for a more complicated codec. I had a Toshiba Libretto 30 with a 486 processor and PCMCIA soundcard. It could play MP3s .. but only with the Fraunhofer codec, Winamp required slightly more than 100% CPU.
- derefr 7y agoThat’s assuming you ran the codec on the CPU. We had codec-accelerator ASICs back then! MP3 players and DVD players were famously built on such ASICs. For a long time, the MPEG-2 decoder ASICs required for DVD playback were a differentiator on video cards and motherboards, allowing some PCs to offer DVD playback long before consumer CPUs were capable of realtime 480p MPEG-2 decoding. (E.g. the original iMac offered DVD playback through such an ASIC.) The real questions in this hypothetical are: 1. if we handed the netlist for a modern H.265 codec ASIC to a 1990s fab, would they have been able to print it? (Maybe.) 2. Would the resulting chip have made H.265 a worthwhile encoding for shipping media in the 1990s? (Nah; the chip, as rendered at a ~100nm process node, probably would have been ridiculously power-hungry and hot. It would have worked out in a server with blower fans, or in a gaming PC with a powerful PSU and a water-cooling rig dedicated to the ASIC; but you couldn’t have put one in a piece of consumer electronics like the PlayStation 1. As such, its use, if anything, would only be studio-internal, maybe for archival storage of masters in a “nearly-losslessly-compressed” form.)
- lonelappde 7y agoIf you're only saw the problem at high frequency, how do you know it was it a problem at low frequency? You concluded there was less common but still fatal bit errors at low frequency?
- kec 7y agoI believe he meant they changed the scale of their test equipment display to more clearly reveal high frequency noise, not that they clocked their circuit higher.
- pjc50 7y agoI read that as adjusting the timebase on the scope; at low-frequency timebase the scope smooths away the noise, but that doesn't mean the digital receiver in the circuit can't see it.
- hermitdev 7y agoYes, this was precisely the issue. We couldnt see the noise at our expected frequency of, say 1 MHz, but we could when scaled to say 50 MHz. Been nearly 2 decades now, so precise numbers elude me.
- pjc50 7y agoThis is one of those things that EE students really benefit from seeing as close to in-person as possible. Especially setups where they can bring in bits of ground plane, fiddle with the termination etc and watch the noise appear and disappear on the scope.