Lower SNR limit of Digital Voice

I’m currently working on a Digital Voice (DV) mode that will work at negative SNRs. So I started thinking about where the theoretical limits are:

  1. Lets assume we have a really good rate 0.5 FEC code that approaches the Shannon Limit of perfectly correcting random bit errors up to a channel BER of 12%
  2. A real-world code this good requires a FEC frame size of 1000′s of bits which will mean long latency (seconds). Lets assume that’s OK.
  3. A large frame size with perfect error correction means we can use a really low bit rate speech codec that can analyse seconds of speech at a time and remove all sorts of redundant information (like silence). This will allow us to code more efficiently and lower the bit rate. Also, we only want speech quality just on the limits of intelligibility. So lets assume a 300 bit/s speech codec.
  4. Using rate 0.5 FEC that’s a bit rate over the channel of 600 bit/s.
  5. Lets assume QPSK on a AWGN channel. It’s possible to make a fading channel behave like a AWGN channel if we use diversity, e.g. a long code with interleaving (time diversity), or spread spectrum (frequency diversity).
  6. QPSK at around 12% BER requires an Eb/No of -1dB or an Es/No of Eb/No + 3 = 2dB. If the bit rate is 600 bit/s the QPSK symbol rate is 300 symbols/s

So we have SNR = Es/No – 10*log10(NoiseBW/SymbolRate) = 2 – 10*log10(3000/300) = -8dB. Untrained operators find SSB very hard to use beneath 6dB, however I imagine many Ham contacts (especially brief exchanges of callsigns and signal reports) are made well beneath that. DV at -8dB would be completely noise free, but of low quality (e.g. a little robotic) and high latency.

For VHF applications C/No is a more suitable measurement, this is a C/No = SNR – 10*log10(3000) = 26.7dBHz (FM is a very scratchy readability 5 at around 43dBHz). That’s roughly a 20dB (100 x) power improvement over FM!

SM1000 Part 13 – Shipping!

The enclosure has arrived from the new manufacturer! Edwin and team at Dragino are now assembling, testing, and shipping the first batch of 100 SM1000s. We plan to ship all Aliexpress pre-orders in week starting 3 May, Australian orders the week starting 10 May.

We have sold almost all of the first batch . . . → Read More: SM1000 Part 13 – Shipping!

Minimalist VHF Software Defined Radio Part 2

Shortly after I published the first post on a simple VHF SDR, Brady KC9TPA started making suggestions about optimising the code. So I encouraged him to have a look into the transmit side. How can we take a baseband modem signal (like GMSK) and convert it up to a HF IF frequency like 10.7 . . . → Read More: Minimalist VHF Software Defined Radio Part 2

SM1000 Part 12 – Testing in the US

Walter, K5WH has one of the 3 pre-beta SM1000 units. He writes:

Here’s a pic of the operations setup of the SM1000 on the air today from Houston Texas, into my HPSDR radio. With average Power down to 3 watts even. Made successful contacts to Mel-K0PFX and Gerry-N4DV. After working the audio levels a . . . → Read More: SM1000 Part 12 – Testing in the US

FreeDV and Codec 2 2015 Road Map

Last week I had a great chat with Gary Pearce KN4AG from Ham Radio Now:

Which brings me to my plans for 2015……..

2015 Open Digital Voice Road Map

I’m pretty excited about where Open Source Digital Radio is going in 2015. My goals for this year are:

A “sub zero” negative SNR FreeDV HF mode.
VHF FreeDV mode(s) that . . . → Read More: FreeDV and Codec 2 2015 Road Map

SM1000 Part 11 – Accepting Pre-orders!

The first batch of 100 SM1000s are being built in China right now and we estimate shipping will start in late March April. Due to popular demand I am accepting pre-orders right now!

Australian customers can buy directly from my Store, rest of the world please use the Aliexpress Store for direct shipping from . . . → Read More: SM1000 Part 11 – Accepting Pre-orders!

Minimalist VHF Software Defined Radio Part 1

I think the future of radio hardware is a piece of wire connected to a GPIO pin.

The rest of the radio will be “gcc compilable” free software running on commodity CPU horsepower. I spoke about this at length in my recent linux.conf.au 2015 talk.

For the last two weeks I’ve been developing a simple radio architecture . . . → Read More: Minimalist VHF Software Defined Radio Part 1

Codec 2 and GMSK over VHF Radio Part 1

In the previous post comparing GMSK modem algorithms, I had some results suggesting we can build a Codec 2 VHF “mode” that outperforms legacy analog FM by 10dB (that’s a factor of 10 in power). It seemed to good too be true. So for the past few weeks I’ve been working with Daniel, VA7DRM, to . . . → Read More: Codec 2 and GMSK over VHF Radio Part 1

SM1000 Part 10 – First Over the Air Tests

Last night I visited Matt, VK5ZM, with a SM1000 Beta. He configured the SM1000 to interface to his IC706 using the CN12 connector patch socket. This allowed an RJ45 cable from the SM1000 to connect to the RJ45 mic/audio connector on the IC706. A few level adjustments and he was using FreeDV to . . . → Read More: SM1000 Part 10 – First Over the Air Tests

GMSK Modem Simulation

Modems are an interface between theoretical physics and what can actually be built. The laws of physics set the limits of modem performance, and ultimately the amount of power you need for a certain bit error rate at a receiver. With the right algorithm, we can reach the limits of modem performance.

I think that’s kind . . . → Read More: GMSK Modem Simulation