freedv – Daniel Estévez

Playing with LilacSat-1

Even though the cubesat LilacSat-1 was launched more than a year ago, I haven’t played with it much, since I’ve been busy with many other things. I tested it briefly after it was launched, using its Codec2 downlink, but I hadn’t done anything else since then.

LilacSat-1 has an FM/Codec2 transponder (uplink is analog FM in the 2m band and downlink is Codec2 digital voice in the 70cm band) and a camera that can be remotely commanded to take and downlink JPEG images (see the instructions here). Thus, it offers very interesting possibilities.

Since I have some free time this weekend, I had planned on playing again with LilacSat-1 by using the Codec2 transponder. Wei Mingchuan BG2BHC persuaded me to try the camera as well, so I teamed up with Mike Rupprecht DK3WN to try the camera this morning. Mike would command the camera, since he has a fixed station with more power, and we would collaborate to receive the image. This is important because a single bit error or lost chunk in a JPEG file ruins the image from the point where it happens, and LilacSat-1 doesn’t have much protection against these problems. By joining the data received by multiple stations, the chances of receiving the complete image correctly are higher.

P25 vocoder FEC

Following a discussion with Carlos Cabezas EB4FBZ over on the Spanish telegram group Radiofrikis about using Codec2 with DMR, I set out to study the error correction used in DMR, since it quickly caught my eye as something rather interesting. As some may know, I’m not a fan of using DMR for Amateur Radio, so I don’t know much about its technical details. On the other hand, Carlos knows a lot about DMR, so I’ve learned much with this discussion.

In principle, DMR is codec agnostic, but all the existing implementations use a 2450bps AMBE codec. The details of the encoding and FEC are taken directly from the P25 Half Rate Vocoder specification, which encodes a 2450bps MBE stream as a 3600bps stream. Here I look at some interesting details regarding the FEC in this specification.

Testing LilacSat-1 Codec2 downlink and GPS telemetry

Today I’ve finally had some time to test the LilacSat-1 Codec2 downlink on the air. I’ve been transmitting and listening to myself on the downlink during the 17:16 UTC pass over Europe from locator IN80do. The equipment used is a Yaesu FT-2D for the FM uplink, a FUNcube Dongle Pro+ and my decoder from gr-satellites for the downlink, and a handheld Arrow satellite yagi (3 elements on VHF and 7 elements on UHF). Here I describe the results of my test.

Low latency decoder for LilacSat-1

LilacSat-1 is one of the QB50 project cubesats. It will be released tomorrow from the ISS. The most interesting aspect of this satellite is that it has an Amateur Radio transponder with an FM uplink on the 2m band and a Codec2 1300bps digital voice downlink on the 70cm band. It is the first time that an Amateur satellite really uses digital voice, as previous tests have only used an analog FM repeater to relay D-STAR and similar digital voice modes. LilacSat-1 however implements a Codec2 encoder in software using its ARM processor. I have talked about LilacSat-1 Codec2 downlink already in this blog. Here I present a low latency decoder for the digital voice downlink that I have recently included in gr-satellites.

LilacSat-1 Codec 2 downlink

LilacSat-1 is one of the satellites that will form part of the QB50 constellation, a network of 50 cubesats built by different universities around the world that will conduct studies of the thermosphere. LilacSat-1 is Harbin Institute of Technology’s satellite in the QB50 constellation, and is expected to launch late this year. Incidentally, his “brother” LilacSat-2 launched in September 2015, and it has become a popular satellite because of its Amateur Radio FM repeater.

Apparently, LilacSat-1 will feature a very novel transponder configuration: FM uplink and Codec2 digital voice downlink. I have discovered this yesterday while browsing the last updates to the Harbin Institute of Technology gr-lilacsat github repository. In fact, there is no mention of digital voice in the IARU coordination page for LilacSat-1. According to the coordination, the transponder will be mode V/U (uplink in the 144MHz band and downlink in the 435MHz band). However, it seems that only downlink frequencies have been coordinated with IARU. Hopefully the uplink frequency will lie in the satellite subband this time. LilacSat-2 is infamous because of its uplink at 144.350MHz, which lies in the SSB subband in the Region 1.

Codec2 is the open source digital voice codec that is used in FreeDV. This makes LilacSat-1 very exciting, because Codec2 is the only codec for digital voice radio that is not riddled with patents. Moreover, it performs much better than its main competitor: the AMBE/IMBE family of codecs, which are used in D-STAR, DMR and Yaesu System Fusion. Codec2 can achieve the same voice quality as AMBE using roughly half the bitrate.

Harbin Institute of Technology has recently published a GNUradio decoder for the Codec2 downlink and an IQ recording to test the decoder. Here I take a quick look at this code and I talk a bit about the possibilities of using Codec2/FreeDV in satellites.

Testing microphone performance for Codec 2

Codec 2 is the open source and patent-free voice codec used in FreeDV, a digital voice mode used in amateur radio. Since Codec 2 is designed to be used at very low bitrates (the current version of FreeDV uses 1300bps and 700bps), it does an adequate job at encoding voice, but can’t encode well other types of sounds, and thus fails poorly in the presence of noise. Hence, microphones which may be good enough for other applications can give poor results when used for FreeDV (if, for instance, they pick up too much ambient noise or have too much echo). This is a small note about how to test the microphone performance for Codec 2.

Listening to the FreeDV net by WebSDR

Since I’m away from my usual QTH, I decided to join yesterday’s FreeDV net by listening through the University of Twente WebSDR and giving signal reports on the QSO Finder to the stations transmitting.