Tag Archives: spectrum

Talking V.23 to a Prism VTX 5000

This is the third post in a multi-part series about viewdata. A few members of the hackspace are working on a project to connect a ZX Spectrum with Prism VTX 5000 modem to an emulation of a viewdata system, providing a live demonstration experience.

A word in advance – this post is quite theory-heavy, and contains quite a bit of technical information. However, hopefully it’s of interest to people. You can find the complete source code for my softmodem at https://github.com/aquila12/v23.

Modems for viewdata systems use Mode 2 of the ITU V.23 standard for communication. This specifies the signal format for encoding the data as transmitted across a phone line. Mode 2 allows a maximum of 1200 baud operation, and specifies the mark (logic ‘1’) and space (logic ‘0’) frequencies as 1300Hz and 2100Hz, respectively. The standard also specifies a backward channel running up to 75 baud, using 390Hz and 450Hz for mark and space, respectively.

Viewdata uses the faster forward channel to deliver data to the customer’s terminal, while key presses are transmitted immediately over the backward channel. With the standard 7-bit ASCII data, even parity bit and the stop and start bits, the frames are a total of 10 bits long – so the forward channel can relay 120 characters per second, with the backward channel running at the much slower 7.5 characters per second. Viewdata frames are up to 40 characters wide, with 24 lines. This divides out neatly so that three complete lines of of non-escaped data can be sent each second (escape codes are used for display attribute changes), with a full frame taking around 8 seconds to transmit.

While a regular V.23 compatible modem could be used, these are somewhat rare now. To communicate with the Prism, we decided to use a software modem. While we’d had some success with minimodem, I found it slightly problematic to use in practice. On my laptop, it used a lot of CPU to demodulate signals, and it struggled to synchronise with the incoming data. I decided to put my somewhat rusty signal processing theory to practice and wrote a V.23-only softmodem to use for this project. My aim was to avoid performing any fourier transforms (e.g. FFTs) on the signal, as these are computationally expensive, and instead stick to simple operations such as moving-average filters and differentiators, as these can be made very computationally efficient.

My first arrangement has limited success, but was too sensitive to noise. The arrangement, briefly, did amplitude demodulation on the mark and space frequencies, then compared the magnitudes to see which was stronger, and fed the output to a shift register which detected the serial frames. Once I’d thought a bit more about the problem, I decided to attempt demodulation using an approach more suited to the frequency-shift-keyed modem signal, which produced much better results. I’ll describe that in more detail here.

Taking an input signal, I multiply on a complex local oscillator which runs halfway between the mark and space frequencies. This essentially moves the frequency spectrum down to DC – anything lower than the oscillator is seen as a “negative” frequency, while anything above is seen as a “positive” frequency – see the image below for a brief illustration. For a complex (I-Q) oscillator, this means that the direction of rotation of the output vector tells you whether the dominant frequency at the input is above or below the local oscillator frequency.

It’s worth mentioning that multiplying on the local oscillator doesn’t just “move” the spectrum – it causes two copies. One is shifted down around DC (the “difference” spectrum), while the other is shifted upwards (the “sum” spectrum). Because of this, and also because the input isn’t completely clean (in fact, it might contain other signals), the signal must be filtered before determining the direction the vector is rotating in. I used a moving-average filter for this.

Signal Processing 101

Signal Processing 101

Moving-average filters are very simple – essentially the samples between two points in time are summed. This is easy to implement efficiently as you only need to keep track of the samples within that period, and keep a running total. As each sample is added to the total, the sample added least recently is subtracted and removed from the buffer. These filters have a very characteristic transfer function (i.e. what frequencies they reject). The moving-average filter completely rejects signals at regular frequencies, and those frequencies occur depending on the period the filter is averaging over – the illustration shows the shape of the response. I used this fact to specifically tune the demodulation filters. When demodulating the forward channel, I placed the first “null” to eliminate the backward channel signal as much as possible (as in the picture), as the backward channel is within the side-bands of the forward channel. For demodulating the backward channel, since the band is quite narrow, I just placed the first null so it was away from the frequencies of interest.

The filtered signal is then passed to a function which determines the current phase of the vector. This essentially divides the vector components to produce the tangent of the phase angle, and then passes that through a function to determine the angle. As the precise angle isn’t important for this, I used a simple approximation for this – the tangent function, between -45° and +45°, is nearly a straight line, and it’s straightforward to replicate this approximation through the entire circle by flipping the division upside down to stay in this quadrant.

Once the phase angle is determined, we just need to average its change over each bit, and use the sign of this (positive or negative) to correctly demodulate the signal. The simplest way to write this was to take the derivative of the phase, and then run this through a moving-average filter. The output is then fed to the same frame-detection routine as I’d drafted for my first attempt, which I’ve illustrated below.

Frame Detection

Frame Detection using a Shift-Register

I’m aware this topic has been quite in-depth as far as the implementation goes, but I’m hoping it makes some sense to those of you who perhaps dabble in filters or radio-related topics! If you’d like me to expand on anything I’ve discussed here, please get in touch and I’ll do my best to address your query.

Next time, I’ll talk a bit about the viewdata frame format and how we’re writing our frames.

Simulating a Phone Line

Line Simulator built in an old ADSL filter box

This is the second post in a multi-part series about viewdata. A few members of the hackspace are working on a project to connect a ZX Spectrum with Prism VTX 5000 modem to an emulation of a viewdata system, providing a live demonstration experience.

Viewdata connections, much like dial-up internet access which followed later, relied on audio frequency telephone communication to transfer data between the user’s terminal and the information provider they were logged into. I’ll cover details of the signal modulation in a separate article; in this article I’ll describe how I connected a real Prism VTX5000 modem to a laptop’s sound card.

A UK telephone line, like that in most countries, is a relatively simple circuit. The line is supplied from a bank of batteries with approximately 50V DC, with earth on the positive side (to prevent line corrosion by offering cathodic protection). This would have originally been supplied via a pair of relay coils, which have three main effects:

  • The coils have a DC resistance, which in practice limits the off-hook condition of the line to about 8V at 20mA
  • The coils present a large inductance, which prevents AC (i.e. the audio signal) from being lost into the supply
  • The relay contacts are used to detect when the line is off-hook

For the purposes of our simulation, we only need the line simulator circuit to behave like a line in the off-hook state. This means it can run from a low-voltage supply of 9-12V. We’re also not interested in providing a ring signal (typically around 90V AC), as it’s also not required for this project.

On a telephone line, the AC audio signal sees a resistance of around 600 ohms. Audio signals are coupled to the phone line via capacitors, which pass the audio signal but block the DC supply.

The line simulator therefore has the following requirements:

  • To provide approximately 20mA at around 8V DC with a telephone connected off-hook
  • To provide several kilohms of dynamic resistance at DC
  • To provide around 600ohms of resistance to AC
  • A means to inject an audio signal from a laptop onto the line
  • A means to take off an audio signal to feed a laptop’s line input
  • Protection to prevent overloading of the sound card

Starting with the DC requirements, a fairly simple transistor circuit can be made to act roughly like a current source, up to near the circuit’s supply voltage. Adding a bypass resistor permits the introduction of a controlled dynamic resistance at DC, and this also affects the off-hook voltage level.

To provide the required AC resistance and injection capability, the circuit can be coupled via a capacitor and series resistor to the laptop’s headphone jack. A second resistor to ground helps to block the DC supply. A headphone output has a very low resistance of a few ohms at most. A 680 ohm resistor increases this to a more suitable level. While this is slightly on the high side, it leaves some room for other parallel resistances of a few kilohms, which the circuit will introduce.

The protection and take-off requirements can be met simply by placing two signal diodes in inverse-parallel; this limits the line signal to 0.7V peak, and also limits the extent of transients when the line switches between on- and off-hook conditions.

Line Simulator Circuit Schematic

Line Simulator Circuit Schematic

The complete line simulator circuit is shown above. The LED and series resistor are used to provide suitable biasing to the transistor. An unintended side-effect is that when the phone is on-hook, the LED supply is bypassed by the transistor, so the LED also acts as an off-hook indicator.

This circuit can easily be built on a small piece of stripboard. ADSL filters are widely available, and often going spare. These provide a suitable project box and, with the rest of the filter components removed from the board, a pair of phone jacks which can be connected to the line simulator circuit. Note that the circuit as presented is not suitable for connection to a PC microphone jack as the signal level is far too high; to use this an attenuator would need to be built in, which could be achieved with a simple resistive divider.

Once constructed, check the voltage at the phone socket is about right, then plug in an old phone to check it’s working. You’ll probably hear some quiet line noise, and the LED should light when the receiver is lifted. If you connect the circuit to a laptop or music player, you should be able to hear audio played clearly through the phone, complete with that lovely tinny quality.

Viewdata – A Brief Overview

This is the first post in a multi-part series about viewdata. A few members of the hackspace are working on a project to connect a ZX Spectrum with Prism VTX 5000 modem to an emulation of a viewdata system, providing a live demonstration experience.

Back in the early ’80s, before the world wide web was a thing, there was something a little like Teletext called Viewdata.

Viewdata pages look a lot like Teletext – they share the same frame formats – and viewdata terminals likely used the same IC to produce the video signal (The Mullard SAA5050, also used in the BBC Micro for its text-only mode). While teletext pages are transmitted in the blanking interval of a TV signal, in rotation so a receiver needs to wait for the right page, viewdata operates over a telephone line and so is more interactive.

When a subscriber wanted to log onto the service, they dialed the provider they wanted to access – for example Prestel – and provided their ten-digit ID number and password to log in.

Once logged on, the user was presented with the main page, and navigated using the number keys and enter. Pages were generally arranged hierarchically, so page 1234 would be below 123, and is reached from that page by pressing ‘4’. Like Teletext, viewdata pages could consist of multiple frames, so pages longer than one frame could be constructed.

Companies who wanted to host information on a viewdata service would pay the information provider (IP); this would involve a payment for a particular length of prefix (shorter prefixes would typically be more expensive), and also per page hosted. They could edit frames using special editing software, and then uploaded these to the IP. Frames weren’t restricted to being completely passive – “submission” frames could be authored, allowing people to order goods or fill in forms. Frames could also be marked with a cost, allowing companies to collect money against a user’s account when they accessed the information.

Often, information providers would have a mailbox system, allowing users to send each other messages, which could be retrieved and read when the user logged in. Some terminals would allow these, as well as other frames, to be retrieved and stored locally, so they could be read offline without the user keeping the line open.

As well as centralised information providers, such as BT’s Prestel, software later emerged allowing people to host their own viewdata systems – often bulletin boards – or set up peer-to-peer connections. Peer-to-peer connections allowed users to phone each other, and as long as they set their modems correctly, information could be transferred half-duplex. Bulletin boards could be run on systems such as the BBC Micro, and would present themselves in the same manner as an IP.

The modem standard – v.23 – used for viewdata is asymmetric. The forward channel (download) runs at 1200 bits per second (120 characters per second taking framing into account), while the reverse channel (upload) runs at 75 bits per second (7.5 characters per second). A terminal – apart from any storage functionality – is relatively “dumb”, and transmits key presses immediately. The information provider’s system reacts as needed – whether this means sending a new frame, or echoing characters typed into a response frame. The slow reverse channel is still generally fast enough to transmit the user’s keypresses, and is fairly noise-resistant because of the low bandwidth.

There are sites which contain demonstrations of some viewdata systems – https://www.viewdata.org.uk/ is a good example of this. Follow the “Logon Now” link to access an example.