Initial design planning

I had the idea for this project a while ago, and came up with the basic project requirements (you can find these on the README in the Github repo). I started properly researching to build the project on the 25th October.
Started by making a mood board-type thing of design inspirations for an idea of what size/control scheme i want to target. I found https://walkman.land/ which is a really cool catalogue of walkman models with high-quality photos and tech specs, go check it out! The design of the TPS-l2 really speaks to me and with the big physical buttons on the side and front flap to load tapes so I want to see if I can combine that with a more ipod-like user-interface for my design.

Before I can do any more mechanical design work I have to get the electronics planned, as that will dictate many aspects of the case. For whatever reason I decided to look for a screen first, but that turned out to be a good decision as after a lot of time on Aliexpress looking at middling-quality LCD screens (shoutout Cheap Yellow Display) I remembered Lilygo make nice devboards with displays built-in.

Scrolling around their website for a bit led me to the T4 S3 which sports a 2.41 inch AMOLED touch screen which is roughly the same size as an iPod classic's LCD, but with more vibrant colours, heaps brighter and almost twice the resolution! On the back side there's an ESP32-S3 microcontroller with 8MB of PSRAM (perfect for audio playback and digital signal processing (DSP)), 16MB of flash, WiFi and Bluetooth low-energy. The devboard also has an SD card slot where I can load my music, a battery charger chip, a usb-c port that supports data transfer and a GPIO header that I'll be able to connect the rest of the audio electronics to.

I did some research and napkin maths to determine that the ESP32-S3 should be able to handle

• FLAC decoding
• 8-ish band parametric EQ filter for headphone correction or shaping the sound to your liking
• A decently complex user-interface All at the same time (if I'm good enough at optimisation. We'll see...). After all it's a dual-core 240Mhz chip which is CRAZY considering it costs under $3 AUD!!

The last thing I did on Saturday was have a look at various forms of physical media to try decide on a form factor for the cartridges. SD cards are great but feel really flimsy and small, I would like mine to feel solid in the hand and be big enough to fit a nice album artwork sticker on. Cassettes are awesome but would make my design way too big + thick. Ultimately I think I'll aim for something similar to a Gameboy Colour cartridge, but a bit thinner and smaller. They have a nice sticker on the front and a grip section at the top to grab them by.

On Sunday I fell down the rabbit hole of audio electronics design (nooooo!). I remembered a team building a similar DIY, ESP32 based mp3 player called the Tangara, and their great writeup on their audio design findings.. They gave an interesting insight into the differences between types of headphones (in regards to sensitivity and fidelity), and how it's hard to design a DAC/Amp combination that works well with both power-hungry high-end headphones and affordable hi-fi IEMs that are very sensitive to noise. I also had a look at what people who know what they're doing have made, like Schiit's DACs (yes that's their name), the FiiO KA1 USB-C dongle that I daily drive, the high-end RME ADI2 DAC, and a whole lot of others.

My conclusion from that was there's a LOT of DAC chips with seemingly very little difference between them as long as you're spending more than $30 and less than like $10k. I settled on the AK4493SEQ as it had decent specs on paper, people seem to like the ADI2 and Schiit Modius that are built around it (actually the ADI2 changed to a similar ESS Sabre chip after the AKM factory burned down in 2020 😳) and I could find it for a good price at LCSC so I can get it assembled by JLCPCB. As for amplification, I'll probably go with the OPA1622 by Texas Instruments because it's powerful, low-distortion, cheap-ish and seems to be pretty ubiquitous (also TI's documentation is always lovely and makes my life easier).

I also spent way too long trying to find a simple bluetooth transmitter chip because people (myself included) love wireless headphones, but unfortunately the microcontroller I'm using doesn't have bluetooth audio support. I decided to forgo this in the first iteration of this design because

1. I prefer the sound quality of wired
2. Wireless headphones are useful in the rare occasions that I want to put my phone down & walk around, but that's usually when I'm listening to a podcast or watching a youtube video so I need the screen visible. This device is a lot more pocket-able so there's less need for bluetooth. Also I know the secret cable coiling technique to stop my earbuds from tangling
3. There are NO easy options for this!! I don't want to waste power with a SECOND entire microcontroller but the only other option seems to be "be an established electronics company" because there's no hobbyist-friendly modules that I can find Ok Bluetooth rant over!

I wasted some time looking at CNC milling material options and surface finishes because I want the player to be nice to hold (this is NOT the time for this I haven't even designed the thing yet omg). I even spent like half an hour researching ceramic spray coats to see if I could get a nice silky-smooth aluminium finish that wouldn't pick up fingerprints (side note I LOVE the etched glass backs of modern iPhones its so smooth but matte at the same time).

FINALLY I did some work researching connectors for the cartridges, I'm torn between magnetic pogo-pin style and spring-contacts like SD cards and camera batteries. I'll think more about this later and maybe do some tests.

Congrats if you made it through my yapping, and I hope you learned something or at the very least found this interesting :P
I'll get to work on the physical design CAD now so I can work out what buttons, pcb layout etc. that I'll need! Gotta lock in for my last few exams first though. Adios!