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Modular Open-Source AV Receiver

A specification and reference implementation for a modular audio/visual receiver

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I'm building a modular open-source AV receiver platform, with the following features
- Receive digital audio from TV via HDMI ARC
- Share control signals via HDMI CEC
- Multi-channel audio decoding and amplification
- Multiple outputs
- Apply DSP effects

Use-cases:
- Allow your TV remote to seamlessly control an existing analog amplifier/receiver
- Send a mono mixdown from your home cinema to the bathroom
- Room/speaker compensation
- Start with a 2.0 system, later upgrade to 5.1

My work-in-progress is available on github. The module specification (electrical, mechanical, and control protocol) will be free for anyone to use, and my module designs will be published as open source hardware (license TBA, likely either GPLv3 or CC-BY-SA, but I’m open to advice).

Background & Inspiration

Audio reproduction technology (by which I mean "speakers") hasn't fundamentally changed for many years. Yet the obsolescence timeframe of an Audio-Visual Receiver is becoming shorter due to changes in digital technology: SPDIF, HDMI, more channels, etc. Newer technologies are increasingly closed-source. My old analogue amplifier still drives my speakers perfectly, why should I throw it out?

Desired Outcomes

The idea is to prevent the wasteful discarding of obsolete equipment by allowing incremental upgrades. The modular system will be entirely open-source so that any interested person can make their own module to serve their unique needs (or for fun). Eventually I would like to see compatible modules available from multiple vendors (much like Raspberry Pi & Arduino add-ons).

Backplane Interface 20170321.ods

Status: Request for Comments initial draft of backplane interface spec, based on PCIe x1 connector

spreadsheet - 16.10 kB - 03/21/2017 at 08:03

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  • 4 × PCA9500 I2C EEPROM & I/O Expander
  • 8 × 74HC157 Logic ICs / Decoders, Encoders, Multiplexers, Demultiplexers
  • 1 × EP91H0 HDMI ARC rx/tx
  • 1 × CS8416 SPDIF decoder
  • 1 × Raspberry Pi As a source of I2S audio

View all 6 components

  • Proof of Concept

    Andrew Bolin05/09/2017 at 10:40 0 comments

    Some good progress over the last few days. PCBs arrived, and I've assembled them - cut down a bit, I left out the bus switches for two reasons:

    1. they aren't needed with only one input & one output.

    2. I didn't have them.

    So the first modules are essentially a weird looking Raspberry Pi add-on. One mistake I did make - no +5V. My backplane spec didn't include 5V because most parts I'm looking at using are quite happy at 3.3V. But maybe that was a mistake. Anyway, easily fixed for now with a piece of wire, and music now comes out of my DAC!

    Next: I2C comms from the Pi to the IO Expander / EEPROM chip that will eventually serve as module ID & control.

  • ARC module layout

    Andrew Bolin04/23/2017 at 06:24 0 comments

    First draft of the HDMI ARC module PCB layout is done. All parts are on the top side, but I did have to use one wire jumper (for the "module enable" signal, very low speed so no problem). It's up on github.

    I realised I could make the ARC chip (Explore EP91H0) function as either transmit or receive by adding a couple of resistors, so I did that. Might come in handy for testing (rather than risking damage to my far-too-expense TV, the only device I have that currently supports ARC).

    I'm using the good old CS8416 for SPDIF to I2S conversion, because I still have one in my parts drawer from my last DAC project.

    The Raspberry Pi adapter boards and backplane are apparently in transit. So, if I'm lucky they'll be here in a couple of weeks...

    Next move: finish the layout of the ARC module (cuts & silkscreen). Might throw in a few guard traces around the high speed clocks if I remember to...

  • Starting the HDMI (ARC) input module

    Andrew Bolin04/11/2017 at 11:32 0 comments

    I'm using the Explore EP91H0 chip, because it's the simplest way I could find to add HDMI ARC (that's all the chip does, no video stuff).

    What's ARC? "Audio Return Channel". When you plug your amplifier to your TV with an HDMI cable, the TV is able to send audio *back* to the amplifier, despite the TV's HDMI socket being an input (in HDMI parlance the TV is a "sink", and the amplifier is supposedly a "source").

    I've also made the schematics a bit modular, using a common sheet for stuff that I expect to be used on most modules (in future I will refine this with further sub-sheets for mandatory / optional bits).

  • PCB Prototype Unit 0 - Design Files

    Andrew Bolin04/10/2017 at 10:59 0 comments

    PCB designs are now up on github, and manufacture is underway, hope to get them in a couple of weeks time.

    Module

    Backplane

  • v0 PCB Production Underway

    Andrew Bolin04/06/2017 at 22:07 0 comments

    After a minor glitch (forgot to export the soldermask layers), the first PCBs are now being made at @Seeed Studio.

    The backplane has four PCIe 1x slots, plus debug headers.

    The modules are just adapters from Raspberry Pi header to my draft PCIe 1x specification. During the process of designing the PCBs I found a few signals were pretty hard to route (I'm only using 2 layers to keep the costs down), so I think I'll re-map some pins and go to a bigger connector for the next version.

    PCB designs are not updated in Github yet, I'll do that this weekend.

  • Starting on a test PCB

    Andrew Bolin03/26/2017 at 06:08 0 comments

    Today I worked on a PCB for proof-of-concept, essentially it will be an adapter to a Raspberry Pi header.

    One module will have a Raspberry Pi, one module will have a Pi DAC module.

    Once this is working, I will be able to use it to check other custom modules.

    So far I have discovered one problem, that using a PCIe x1 connector probably isn't going to allow me to use more than one signal source. Pretty much all the pins are used up with 8 channels of audio, all using the same clocks. I suspect it will be impractical to sync multiple sources with the same clock (although I will look into it), so I will probably have to go to a bigger connector to allow multiple sets of 8 audio channels. Maybe I'll go for a few sets of stereo and a couple of 8 channel sets (or is stereo falling out of favour?)

    Schematics are on github, unfinished, not laid-out yet.

    Next: Finish schematic, layout, send for manufacture.

  • Design thus far

    Andrew Bolin03/21/2017 at 00:56 0 comments

    A summary of my design thus far (work very much in progress)

    - 8 channels of digital audio (considering upgrade to 16 or more)

    - Backplane with PCI-express connectors (cheap connector with many pins, saves cost on module)

    - Daisy-chain modules to allow DSP of any input (or no DSP module at all)

    - Module capability discovery via I2C/SPI EEPROM

    - Minimal work required of CPU (e.g. choice of Arduino, Raspberry Pi, maybe 6502 if you can bit-bang the bus)

    Vague block diagram:

    (signal flow is from left to right, my drawing tool was too tedious to add arrows)

    Control module may or may not also act as an input module (signal source).

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