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  • palmerr's Avatar
    06-22-2020, 06:58 AM
    palmerr replied to a thread Mounting issues in Audio Projects
    Your "a little bit of stiff wiring" should be OK. An inch or two is a reasonable limit for unshielded wire. The mic cable can be yards long without affecting quality, when shielded cable is used.
    1 replies | 169 view(s)
  • palmerr's Avatar
    06-16-2020, 02:24 AM
    Yes, one set of pullups for each I2C bus. The best value depends on the I2C frequency, rail voltage and the bus capacitance (related to the wire length). As a rule of thumb for 400kHz operation, 2mA is about right...
    4 replies | 136 view(s)
  • palmerr's Avatar
    06-16-2020, 12:18 AM
    As I2C is an open drain bus, it will float up to whichever supply the pullup resistors are attached to. As Paul provides pullups on the Audio Board, attached to the 3.3V rail, you should be OK. Just check the...
    4 replies | 136 view(s)
  • palmerr's Avatar
    06-11-2020, 11:33 PM
    A compressor would be a worthy addition to the audio library. I have working code for a soft/hard limiting compressor if you would like a place to start. The existing project uses a gain cell to compress analog audio...
    6 replies | 182 view(s)
  • palmerr's Avatar
    06-08-2020, 01:53 AM
    Hi Nevin, A quick read of the BOM looks OK, but here's one directly derived from the schematic for comparison. (It's from the standard BOM ULP in Eagle). The diode types are outlined on the schematic: D1-4:...
    68 replies | 1670 view(s)
  • palmerr's Avatar
    06-06-2020, 12:36 AM
    Yes, you can do that, but better to get the volume levels right in code - to maximise the number of bits available, and therefore noise and distortion. The Headphone output also has a maximum gain of 12dB (Av = 4). ...
    6 replies | 182 view(s)
  • palmerr's Avatar
    06-04-2020, 06:19 AM
    Yep, it needs to be completed well within a 2.9 mS window (44.1KHz / 128 samples) and leave plenty of time for other processing. https://www.pjrc.com/teensy/td_libs_AudioProcessorUsage.html update() is triggered...
    10 replies | 237 view(s)
  • palmerr's Avatar
    06-04-2020, 06:11 AM
    Not really enough free pins on most Teensy projects for even an 8-bit parallel interface! The only 3.5" SPI/SPI TFT boards I could find were designed for the RPi - and this library works OK with that. Here's a...
    26 replies | 8429 view(s)
  • palmerr's Avatar
    06-04-2020, 06:05 AM
    Good to hear that you got it going.
    10 replies | 237 view(s)
  • palmerr's Avatar
    06-03-2020, 02:21 AM
    I'd be inclined to do the output connections separately, as on a single board layout, you are constrained to "in one line" layouts and the board would get quite long. There's really no need for a PCB, just mount them...
    68 replies | 1670 view(s)
  • palmerr's Avatar
    06-03-2020, 12:26 AM
    Hi, Unfortunately the Ethernet code, and particularly the WIZ routines, are not re-entrant and particularly do not fare well if any (software or hardware) interrupt-driven code tries to use the same SPI channel. ...
    10 replies | 237 view(s)
  • palmerr's Avatar
    06-03-2020, 12:06 AM
    Hi, My two ILI9486 libraries are modifications of the Adafruit ILI9341 code - one generic and one based on the ILI9341_t3 Teensy-optimised code. From memory, the only changes I made were in the initialisation, so...
    26 replies | 8429 view(s)
  • palmerr's Avatar
    06-01-2020, 02:04 AM
    As for ground plane design, the basic tenets of mixed signal ground plane design are well-covered in this app note from Analog Devices. https://www.analog.com/en/analog-dialogue/articles/staying-well-grounded.html
    6 replies | 161 view(s)
  • palmerr's Avatar
    06-01-2020, 01:59 AM
    Sure, any audio op-amp could be used. Here is a common configuration. It's relatively simple in concept, but needs a reasonable number of components to ensure the circuit is stable and add some low-pass filtering...
    68 replies | 1670 view(s)
  • palmerr's Avatar
    05-30-2020, 01:46 AM
    Hi Nevin, Doing well thanks. Your design is pretty stable for now, (files on my github) and I'll make any changes required if the spec evolves further. I'm currently distracted with an ESP32 digital power...
    68 replies | 1670 view(s)
  • palmerr's Avatar
    05-28-2020, 01:20 AM
    The simplest solution may be to move to the T4.1 (or any T4), which has accurate 44.1kHz sampling rates. I got caught with dropped frames in the Ethernet library when transferring samples between T3's and T4's due to...
    11 replies | 266 view(s)
  • palmerr's Avatar
    05-26-2020, 12:52 AM
    The CoolAudio chips are definitely the way to go. I believe the ADAT timing issues have been resolved on the T4 - which has an exact 44100 clock frequency. So ADAT output will probably sync nicely. ADAT input is...
    12 replies | 1642 view(s)
  • palmerr's Avatar
    05-26-2020, 12:38 AM
    Just to add to the layout discussion, small component spacing is a pain for home builders but also fabricators. Keeping the analog and digital ground planes separate, and joining them up under the CS42448 is best...
    46 replies | 2931 view(s)
  • palmerr's Avatar
    05-20-2020, 11:43 PM
    Thanks for the heads up. I'm keeping to things that are in the core of the Teensyduino library at the moment, but will watch the FNTE/Native Ethernet developments with interest and definitely provide that option when...
    44 replies | 1808 view(s)
  • palmerr's Avatar
    05-20-2020, 06:45 AM
    In what way? The core of the 4.1 is identical to the 4.0, so the basic code should be OK. If you wish to use the extra SPI pins - SPI remapping is already in place. Are there other features that you can see might...
    44 replies | 1808 view(s)
  • palmerr's Avatar
    05-17-2020, 06:37 AM
    The +/- 12V boost module works OK down to 3.2V with a 100Ma load on one leg. At 3.0 V input the output is down to 7.5V, which is still adequate for our needs. I don't have one of the phantom boost modules in hand...
    68 replies | 1670 view(s)
  • palmerr's Avatar
    05-17-2020, 05:57 AM
    There's no real reason why it won't work, but the current draw from 3.3v would be nearly twice that from 5V. I'll try the PSU modules out on the lower supply voltage and see!
    68 replies | 1670 view(s)
  • palmerr's Avatar
    05-17-2020, 05:54 AM
    Try the files in my development repo https://github.com/palmerr23/audio They have some updates that make them behave better with certain other modules (SGTL5000 and CS2448 specifically), but are not fully tested...
    44 replies | 1808 view(s)
  • palmerr's Avatar
    05-16-2020, 02:29 AM
    Sleeping on it provided the answer: - Go back to a simple switched gain arrangement for the THAT chip 20/40dB. - The pot controls the gain (0-20dB) of the final stage. Let's try and finalise things this week -...
    68 replies | 1670 view(s)
  • palmerr's Avatar
    05-15-2020, 07:46 AM
    Good to hear that your T4.1 is on the way! I've been trying to sort out the gain issue that mhelin identified in my spare moments this week. So far I haven't found an elegant solution, as the THAT chip really...
    68 replies | 1670 view(s)
  • palmerr's Avatar
    05-11-2020, 03:36 AM
    The unity DC gain idea is a good one on the op amp. I've been batting around which taper would be better. THAT suggests reverse audio (log) tapers for the pots, as you note, so I'll start with C (US) taper and see...
    68 replies | 1670 view(s)
  • palmerr's Avatar
    05-09-2020, 06:05 AM
    Yes, I got a fairly good outcome with a dual ganged A taper pot. The Bourns PTD90 series pots are the same width as the original, so no change needed to the board outline. Here's the curves from the simulation: ...
    68 replies | 1670 view(s)
  • palmerr's Avatar
    05-08-2020, 07:34 AM
    We already have separate phantom power for each channel, with switches S1 & S2. The discussion @Mhelin and I have been having is about overload on the microphone preamps with very loud inputs (such as close-miked...
    68 replies | 1670 view(s)
  • palmerr's Avatar
    05-08-2020, 03:50 AM
    BTW, with the multi-coloured LEDs, Focusrite uses them for general gain adjustment: - off = very low signal - green = signal > -24dB(FS) - i.e. level is OK if this is coming on with the signal - red = clipping
    68 replies | 1670 view(s)
  • palmerr's Avatar
    05-08-2020, 03:36 AM
    Nevin, We could achieve the desired result with a switch / pot - Bourns PTM90 series, for instance. The switch would provide 20dB less gain on the mic circuit These are 9mm wide, so no difference to board dimensions,...
    68 replies | 1670 view(s)
  • palmerr's Avatar
    05-08-2020, 12:16 AM
    I eat my words! (and will re-think the front end of the design to avoid potential front-end overload). Thanks for being persistent in correcting my rookie error - one of the side effects of not touching a discipline...
    68 replies | 1670 view(s)
  • palmerr's Avatar
    05-07-2020, 07:34 AM
    Nevin, I'm thinking of using the WS2812 idea on the 8 channel board, as it saves 7 GPIOs. Going with normal LEDS on this project makes good sense, as one GPIO (particularly on a T4.1) is not a good trade-off for the...
    68 replies | 1670 view(s)
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