GB2140642A - Programmable spectrum shaper - Google Patents

Abstract

A programmable spectrum shaper, used for example in the generation of test signals for testing telephones, uses a number of filter modules, each including three one-third octave filters (FILTER 1, FILTER 2, FILTER 3), each with its own adjustable attenuator (A1) and buffer amplifier (FBA). There is output bus (ASB) fed by the summed outputs of the FBAs. The analogue input signal is from an input buffer amplifier (IBA) which applies its outputs to all of these filters. A controller module in response to digital input signals selects the filters to be used and their respective attenuations. <IMAGE>

Description

SPECIFICATION Programmable Spectrum Shaper This invention relates to a so-called spectrum shaper, the function of which is to shape a signal to a specified spectrum. Such spectrum shapers are used in the generation of signals used to test telephone applications.

According to the invention, there is provided a signal spectrum shaper or equaliser system, which includes a number of filter modules each of which includes a plurality of one-third octave filters each with its own adjustable attenuator and buffer amplifier, an output bus to which the outputs of said filters are connected via respective ones of the buffer amplifiers, an input-output module which includes an input amplifier via which an input analogue signal is applied to said filters via the respective ones of said attenuators, and a controller module which in response to a control input generates output data words which define the filters to be used and the attenuation required for each filter's portion of the sound spectrum, so that an analogue input signal can be subjected to a desired pattern of attenuation.

An embodiment of the invention will now be described with reference to the accompanying highly schematic drawing.

The instrument comprises essentially a set of parallel one-third octave filters, input and output buffer amplifiers and a device controller. The various units which make up the instrument are described below.

(1) Input-Output Module The input buffer amplifier IBA is a very low noise device (3nV/root Hz at 1 kHz), and has software-selectable gain by the GAIN SELECT block, to make best use of the available signal-tonoise ratio of the whole system. It also provides a low output impedance to drive the analogue signal bus ASB. This supplies the input signal to all filter modules.

The input amplifier IBA is followed by a signal overload detector OD which monitors the input signals to the filters and signals the controller module via the CONTROL BUS if the signal handling capability of the filters is exceeded.

The output amplifier OA sums the outputs from all the available channels and provides a low output impedance.

(2) Filter Modules Each filter module contains three individual filters, FILTER 1, FILTER 2 and FILTER 3, each with its own software-controllabie attenuator such as Al and buffer amplifier such as FBA. Each attenuator may be set to any value in the range 0 to 88dB and may also be set for infinite attenuation. The card addressing (user selectable by dual-in-line switches) is so arranged that the three 1/3 octave filters may be set and used independently or combined as one octave filter (see section on programming). All the data is latched, so each channel is fully independent unless addressed as an octave filter group.

(3) Controller The controller module handles all the signal protocols, data handshaking etc., performs the separation of card address and card data, and generates the necessary control signals such as data write pulse' etc. This module also generates an SRQ (Service request) signal in response to an overload indication from the input/output module.

Dual-in-line switches in this module enable user selection of the filter address, and also 'mode' selection for the controller. Mode selection enables the device to be set to high or low speed operation or to select 'listen only' operation etc.

(Normal operation would be high speed talk and listen).

(4) Programming Each filter (one-third octave) may be individually addressed by a six bit code as follows.

The two least significant bits select the individual filters or thier combination as an octave filter, while the other four bits are the card address, which is user selectable on dual-in-line switches.

(MSB) AS A4 A3 A2 Al AO (LSB) Card Address Filter Address Al AO 0 O LF filter 0 1 MF filter 1 0 HF filter 1 1 Combined as octave filter.

To set a filter, two bytes of data must be sent, the first byte being a card and filter address and the second being the attenuation required.

Attenuation is specified in terms of 0.375 dB steps.

As an example, assume the card address is 0100 (4) and we need the MF filter to be set for 12 dB attenuation. We send the two consecutive bytes 1710 and 32,0, as follows: (i) 0100 01 =1710 Card 4 Filter (ii) 12 dB/O.375=32 If we need to set many filters at the same time, the data bytes should be sent sequentially, i.e.

card, atten, card, atten, card, atten....

If an octave filter (3) is specified, the attenuation set is the same for each of the filters on the card overriding any previous setting.

The date sent in this way must not be terminated with a Carriage Return or Line Feed, etc.

Control of the preamplifier is similar, but card address 0 is used and the following data byte specifies the gain in terms of steps, which in one case are 6 dB steps; note that at "power-up" preamplifier gain is OdB.

To ensure that the instrument responds correctly to the 1 st byte sent it is desirable that a Selected Device Clear (SDC) command is sent before any data.

If the instrument detects an input signal which exceeds the present signal peak, in this case +/~ 7 V, it generates a Service Request (SRQ) signal.

This feature may be disabled if required by a link on the controller board.

The advantages of such an arrangement are as follows: (1) Ease of use as it is fully software controllable. With current manual spectrum shapers, obtaining a desired spectrum to any reasonable accuracy is time consuming, due to interaction between adjacent bands. With this instrument the whole job can be handled by a computer with minimal effort.

(2) Versatile as any user-selectable combination of filter modules, up to a maximum of 12 (i.e. 36 individual filters), may be plugged into the unit to cover some or all of the frequency bands from 10 Hz to 40 kHz.

(3) Octave-1/3 octave: each module may be addressed as three discrete filters or as a combined unit (i.e. the three 1/3 octave filters produce a standard octave filter).

(4) Other standard filters and weighting networks are available. At present they comprise A, B, C and psophometric weighting curves.

(5) Signal overload detector readily generates SRQ in response to filter overload.

(6) Large dynamic range as it has an 88dB attenuator range, plus in one case a 20dB range of input amplifier gain.

(7) Easy maintenance as it uses fully modular plug-in construction, ensuring easy repair and replacement.

Claims (3)

1. A signal spectrum shaper or equaliser system, which includes a number of filter modules each of which includes a plurality of one-third octave filters each with its own adjustable attenuator and buffer amplifier, an output bus to which the outputs of said filters are connected via respective ones of the buffer amplifiers, an inputoutput module which includes an input amplifier via which an input analogue signal is applied to said filters via the respective ones of said attenuators, and a controller module which in response to a control input generates output data words which define the filters to be used and the attenuation required for each filter's portion of the sound spectrum, so that an analogue input signal can be subjected to a desired pattern of attenuation.

2. A system as claimed in claim 1, and in which each said module includes three one-third octave filters, which can be programmed for use as a single octave filter.

3. A signal spectrum shaper or equaliser system, substantially as described with reference to the accompanying drawings.