Sunday, March 14, 2021

QRTape | Audio Playback from Paper Tape with Computer Vision

Here is a project that I have been tossing around in my head for at least a year or two now. Given advances in audio compression algorithms and computer vision: could reasonably high-quality audio be stored on a paper tape?

This is a fascinating concept to me. When considering the complete history of storage media from early cylindrical engraved records through to magnetic tape it is evident that achieving high quality and reliable data storage is both challenging and expensive. When specifically considering magnetic tape, the challenges surrounding complex mechanical tape transports and sensitive electronics highlight some of the difficulties that engineers have faced in the past.

In this blog I demonstrate a system that exploits modern computer vision and audio compression to replace the complex mechanical tape transports of the past. In fact, my tape transport is made entirely from paper and cardboard (excluding some electronics, of course). I have decided to call this system QRTape.

QRTape Player
Under the QRTape system, data is encoded by a series of QR codes that are printed on a continuous strip of paper. This strip of paper is fed from one spool through a crude tape transport, past a webcam and onto a take-up spool. The paper is advanced by a small stepper motor driven by a cheap Arduino. The rest is pure software magic.

Monday, November 30, 2020

nerfnet: Streaming Video over nRF24L01

A couple of days ago I published a blog discussing how I used NRF24L01 radios to implement a point-to-point network between two Raspberry Pi computers. I implemented this as a virtual network device and sent packets between the radios.

Since then, I have made numerous improvements to the software and more than tripled the throughput from ~90kbps to nearly 300kbps. These improvements were through a variety of changes that I will cover in this blog post.

Streaming video from one headless Raspberry Pi to another

Thanks to the higher throughput, I was able to implement streaming video using the h264 HEVC video codec and monaural audio using the Opus codec at 32kbps. The result is great, especially when considering the link.

Continue reading to learn more!

Friday, November 27, 2020

nerfnet: Wireless Networking over nRF24L01 2.4GHz Radios

I recently picked up a set of nRF 2.4GHz radio transceivers. These are low-cost radios with a SPI interface that allow exchanging 32 byte packets across a radio link that can run at up to 2MBit on-air data rates. They are popular among hobbyists who want to introduce wireless to their Arduino-flavored projects. I was able to buy ten of these radios with trace antennas for just $11 and three more with SMA-connected antennas for $18.

NRF24L01 Radios

My first inclination is to try something a bit more extreme with this hardware. There is a GitHub project named RF24Audio that allows transmitting audio data over these radios. I wondered if video could be possible and started brainstorming about how a video transport over this link would look. The further I got into the specifics of streaming video the more convinced I was that an abstract link that could carry any form of data would be more fun.

This led me to build nerfnet: a simple application that allows sending network frames over NRF24L01 radios. This is implemented by exploiting the TUN/TAP virtual network device API under Linux on a Raspberry Pi. The code is available on GitHub for you to review and use.

I was able to demonstrate nearly 90kBit throughput as measured by iperf. I suspect this is the first time that iperf has been used to characterize a link composed of these radios.

andrew@andrew-pi:~/Projects/nerfnet $ iperf -c
Client connecting to, TCP port 5001
TCP window size: 43.8 KByte (default)
[  3] local port 34490 connected with port 5001
[ ID] Interval       Transfer     Bandwidth
[  3]  0.0-10.1 sec   110 KBytes  89.4 Kbits/sec

Continue reading or watch the video to learn more about how I pulled this off.

Friday, September 25, 2020

Plaid Model S Excitement

If you have been following my blog for a while, you probably know that I am an unabashed Tesla fan. My heart lies with the Model S, the only car that I have ever bought twice.

The recent Laguna Seca record-setting lap has me excited. One thing that stood to me was the e-stop stuck to the dash. I wonder what it does. Does it cut the main battery pack contactor? Wouldn't it be too late by the time it needs to be disengaged? Does it dump fire suppressant over the vehicle? Well, I might be too far out of the loop to ever know what function it performs but decided to re-create the look for fun.

The Resistor Network Plaid Model S :]
The total cost was $15 for a pair of dubious e-stop switches and a couple of small squares of 3M VHB. I put on my longboarding attire and snapped a few photos with my new Canon R5. After a little futzing about in Adobe Lightroom, I ended up with this.

My blog has been a little light on projects lately. Work definitely keeps me busy. It is fun to head out and be creative every once in a while though. This was taken near The Computer History Museum in Mountain View, California. There is a Supercharger nearby.

I hope you enjoy my fun and congratulations to Sebastian Vittel on the impressive lap!

Tuesday, July 28, 2020

Amateur Radio from 2900ft. on Mount Diablo | 146.520MHz FM Simplex

Over the past weekend I took a solo, socially-distanced road trip to Mount Diablo to try out my Amateur Radio station. The results far exceeded my expectations and I ended up making more than 25 contacts total.
View from Mount Diablo with Antenna on Trunk
I decided to put together a video about my adventure. It turned out to be a nice day and a good way to get out of the house. The Amateur Radio Community is incredibly friendly which made for many smiles and fun contacts.

I decided to QSL as many of these contacts as possible. I hope to get some interest cards back from the various operators that I was able to reach.
First QSL Card Design: Lick Observatory
Thanks for reading and I hope you enjoyed the video :]

Tuesday, February 12, 2019

datvideo: Storing Video on Digital Audio Tape (DAT)

About a year ago I added a Digital Audio Tape (DAT) deck to my home theater system. I was turned onto the format by popular Youtuber Techmoan and a great video that he produced: Digital Audio Tape: The one DAT got away. I had wanted to add magnetic tape to my home theater for some time and this highly unique format seemed like a perfect choice.

Big Buck Bunny from DAT Tape
Shortly after purchasing the deck I realized that I could likely store any arbitrary binary data on the tape with the S/PDIF input/output. It would be really cool to merge some modern high-compression video codec with this antiquated format. Over this weekend I decided to do just that.

I wrote a small tool called datvideo that allows storing arbitrary binary data on the tape. This tool is used to grab raw audio binary data from a sound card, search for frames of binary data, decode them and emit them into another file. This can be assembled into a pipeline to feed video data into a player such as mplayer. How cool is that?

Sony DTC-690, below Marantz Blu-Ray, AV Receiver and New-Old-Stock DAT Tapes
In this article, I will walk you through how I pulled this off.

Tuesday, December 4, 2018

PortL2 - Portable Electric Vehicle Charger

Over the past year I have been tossing around the idea of extending the electric range of my Cadillac ELR. I have wanted to build a large scale lithium battery system for some time and this seemed like a great way to learn about the technology.

I had originally started with the idea of charging the ELR hybrid pack while driving. The plan was to use a 48V to 390V (96S) lithium battery charger. Accessing the high voltage bus in the car proved to be challenging and made this approach too difficult for my taste. This made me decide build a portable L2 charger using a 240V inverter and an OpenEVSE instead.

Cadillac ELR Charging at Sierra Trail Head, No L2 Chargers for Miles
I used four battery modules from an old Enginer PHEV Conversion Kit, with a design capacity of more than 2kWh each. The total capacity is around 7kWh now as the cells are several years old and have seen some use. The results so far have been great with the system capable of charging the car to more than 60%.

In addition to being a great portable car charger, this has proven to serve well in powerwall applications. I am able to shift on-peak loads to off-peak, thus saving money and reducing reliance on dirty sources of power.

PortL2 Before Transferring to the Car
I used an Orion Jr. BMS to monitor the pack, perform cell balancing and monitor temperatures. The video above has a complete build log and you can find more details in the rest of this article.

Sunday, February 18, 2018

Groking the Cadillac ELR

Those who know me well know that I have a passion for electric vehicles (EVs). I own a Tesla Model S, have driven it across the US and Canada twice, and am currently designing an electric bicycle with design cues taken from larger EVs and based on lessons learned from my electric skateboard project. I really can not scratch the itch to drive and learn more about EVs.

Last fall I purchased a Cadillac ELR to keep my Tesla company in the garage. I am a huge fan of this car for the striking good looks and the fact that it is a 2-door coupe. I had also never experienced a PHEV (Plug-In Hybrid Electric Vehicle) and really wanted to try out the GM Voltec platform. The prices of the Cadillac ELR are plummeting due to their relatively short-lived time on the market and the fact that they are a little unknown in the eyes of the average consumer.

Cadillac ELR at Cardinale GMC in Seaside, California
I did not buy this car with the intention of merely letting it sit in the garage though. One of the great features of the Tesla Model S is that it lets you "nerd out" with all kinds of stats about your trip. You get total energy consumption in kWh, efficiency in Wh/mile, range estimates based on change in elevation/speed and a whole lot more that makes driving a true delight.

The Cadillac ELR (and Chevy Volt - they share the same powertrain and a similar UI) tend to shield you from these details. I wanted more and without having to do quick mental math based on the limited information available from the built-in infotainment system.

Speed, Distance, Total Energy and Wh/mi Top | State of Charge (SoC) kWh Left | Speed mph Right
I spent about a week decoding traffic on the CAN bus of the ELR in an effort to find a few signals: speed, state of charge (SoC) in kWh, and odometer or a trip of some sort. Once I found these fields, I built a UI with PyQt to render statistics in real-time.

CAN Bus Interface Cable
I packaged it all up into a neatly organized cable and used a Sony Vaio UX to read data from the CAN buses and visualize it on a small display. Continue reading for more details.