CA1186380A - Method for transpoting sub-bands - Google Patents

Abstract

ABSTRACT
The invention relates to a method for transposing sub-bands of equal width in a signal-frequency band by means of modulators followed by equal band-pass filters and additional modulators, characterized in that specific carrier-frequencies are permanently assigned to the modulators, and in that arranged between the band-pass filters and subsequent modulators is a switching matrix for scrambling the sub-bands. The method according to the invention has the advan-tage that there is no need to switch over between the carrier-frequencies fed to the modulators and this produces a saving in circuit costs. In a further devel-opment of the invention, the pass-band of the band-pass filter is applied to one of the sub-bands, thus dispensing with two modulators and the production of a carrier-frequency. According to further configurations, favourable values are given for scrambling a CCITT speech-channel.

Description

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This invention relates to a method for transposing sub-bands of equal width in a signal frequency band by means of modu-lators followed by equal band-pass filters and additional modula-tors.
Such methods are known, for example, from German Patent 24 26 451 which describes a circuit arrangement for obtaining fre-quency scanning, independent of any transportation, for the harmon-i.cs of basic speech-frequencies in sub~band transposition devices, wherein the speech-signal-band is divided, at the transmitting end, into sub-bands of equal width and a transmission~band is formed by transposing the sub-bands, the sub-bands contained therein being adapted to be inverted and the said transposition being reversed at the receiving end; the said circuit arrangement being characterized in that the ratio between the frequency and the lower limit of the speech-band and half the width of the sub-band being as two whole numbers to each other. This circuit arrangement comprises five modulators which, by means of suitably selected upper carrier-frequ-encies at the outputs from the subsequent unit-filter, break the speech~band down into five sub-bands; with the aid of five more sub~
sequent modulators, the sub-bands thus obtained are converted back into the original frequency-position in the speech band, and trans-position of the sub-bands is achieved in that the modulators are supplied with a selectable choice of five out of ten carrier-frequ-encies.
DE-OS 26 52 607 describes a procedure for the concealed transmission of information bearing signals, in which the known processes of time scrambling and frequency band switching are inte-,, ,','!, ' 1 ~

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grated. Here, too, -the costs involved for ring-type modulators, band filters, memory matrices, frequency generators and control are considerable in the analogue version~ In the digital version, which operates according to the Weaver phase method it is true that there is only one input and one output multiplier, but these operate at high multiplication rates and they obtain their single carriers from a sine/cosine table.
U.S. patent 2,408,692 describes a transmission system that involves concealment by sub-band transposition in the frequ-ency range of one of the sub-bands and subsequent transposition into length-modulated pulses that are sent in time multiple.
The known band-pass method involves relatively high circuit-costs~
It was therefore the purpose of the present invention to provide a method of sub-band transpos.ition of the kind mentioned above wh.ich can be achieved at less cost and permits discontinuous ox dig.ital signal-processing.
The mekhod according to the invention .;s characterized in that specific carrier-frequencies are permanently assiyned to the modulators and in that arranged between the band-pass filters and subsequent modulators is a switching matrix for scrambling the sub-bands.
The method according to the present invention shows how the frequency values for the carrier oscillations are to be selec-ted, depending on the division into sub~bands, as well as the carrier frequency, so that a time-discrete or digital signal processing can be carried on in an inexpensive manner. In this

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connection, both s~uare as well as sinusoidal carriers can be used at relatively low scanning frequencies and with simple modulator circuitry. In a further form of the invention, values that are favourable for the encryption of a CCITT speech channel are cited, this being divided into five sub-bands.
The method according to ~he invention has the advantage that there is no need to swi~ch over between the carrier-frequencies fed to the modulators and this results in a saving in circuit-costs.
In a further development of the invention, the pass-band of the band-pass filter is applied to one o~ the sub-bands, thus dispensing with two modulators and the production of a carrier-frequencyO
~ccording to further configurations of the invention, information is given for selection of frequency-values for carrier-waves as a function of division into sub-bands and of scanning frequency, thus making it possi~le to carry out discontinuous signal-processing in an advantageous manner. In this connection it is possible to use both square and sinusoidal carriers at relatively low scanning frequencies and simple modulator circuitry. According to still ~urther configurations of the invention, favourable values are yiven f~r encoding a CCITT speech-channel (CCITT= Comite Consultatif International Téléphonique et Télégraphique) which is divided into five sub-bands.
The invention is described hereinafore in conjunction with the drawing attached hereto, wherein:
Figure 1 shows a block-diagram of a system according to the invention for encoding speech by means of five-band transposition;
Figure 2 shows spectra according to fre~uency-inversion 2a

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with four sinusoidal carrier-waves and discontinuous si.gnal proces-sing;
Figure 3 shows an example of a modulator arrangement;
Figure 4 shows spectra according to frequency-inversion with a square-wave carrier and con~inuous signal processing.
The basic block circuit-diagram of a speech-encoding sys-tem based upon five-band transposition is shown in Figure 1. The input spectrum of speech-signal Xin~f) to be encoded may be burden-ed with unwanted amounts of - 2b -~.~8~38~

higher-frequency spectra which must be suppressed, possibly by a low-pass fi]ter TP 1 at the input-end so that, for example, with discontinuous signal processing, with predetermined scanning frequency fS, the scanning theorem may be maintained. The resulting band-defined spectrum F(f) is divided, in the next step, with the aid of similar band-pass filters, into five sub-bands. This requires prior modulation with appropriate carrier-frequencies ~ 2 etc..
'I'he actual scrambling takes place in the next step by means of a swîtching (scrambling) matrix M controlled by a random-number generator ZG, sub-bands Xv (f) v = 1, 2 ... 5 being transposed at random in short time-intervals accor-ding to (i, j, k, 1, m)C~l, 2, 3, 4, 5). Sub-bands ~Xv ~f) for v = i, j, k, 1, m) are then transposed again with the same frequencies as the input-modulators and thus appear in scrambled sequence in the original frequency-range of speech-spectrum X(f). Possible harmonics of the output-spectrum, for example un-wanted products of modulation or parts of the spectrum recurring periodically during discontinuous signal processing, are eliminated by low-pass filter TP2.
Y(f) is the spectrum of the encoded continuous output signal. The system for decoding encoded speech signals also has the block-structure shown in Figure 1 but, or correct decoding, random-number generator ZG at the receiving end must have the same random sequence as the transmitter and must be synchronized therewi~h.
The m~thod according to the invention also makes it possible to dispense with two modulators if the pass-band of unit band-pass filter BPl to BP5 is applied to one of the five sub-bands and if one of the carrier-frequencies at the input and output-end receives the value 0.
Figure 2a shows the speech-signal spectrum (telephone quality), the normal position of the continuous signal being fully represented and the addi-tional parts of the spectrum appearing symmetrically with whole-number multiples 3~C3 of scanning frequency fS (here the inverted position is shown below fS) during discontinuous processing.
The CCITT speech-channel is established within the limits of 0.3 to 3~4 kHz and, as a result of the division into five sub-bands, sub-band width B = 620 ~Iz. The arithmetical mean frequencies of the sub-bands are then:
fM~' = (2v - 1) . 2 ~ 300 Hz (1) for v = 1 to 5. In the case of simple carrier-frequency production, and for discontinuous signal-processing, it is desirable to modify the mean frequencies slightly with:
fM = v . b (2) The speech-spectrum to be scrambled is thus extended from B2 to 112-(0.31 to 3.41 kHz).
In order to transpose the sub-bands of the spectrum acc-ording to Figure 2a, pertaining to the continuous signal, to the same fre~uency-band by means o~ square wave carriers, basic frequ-encies fv for v = 1 to 5 must be arranged e~uidistantly at inter-vals B. ~ccordiny to the formula:

~v (nv ~ 1) . B
for nv = ~ 1, 2 etc. v = 1 to 5, 1 = ~ IN and i = 0, 1, ... , 1 - 1 as freely selectable parameters. In the simplest case one obtains fv = nv . B for 1 = 1 and i = 0. In this case, the minimal value of the square wave carrier frequencies must not exceed 4~, other-wise spectral overlapping occurs as a result of the Hermiticity of the spectra and this interferes with the signals. This condition also appears as a result of the line-spectrum of the square wave carrier according to Fourier. The above-mentioned restriction does

- 4 -not apply for nv = corresponding to fv = ~ since in this case there is no frequency-inversion. The overlappings are clear to see in Figures 2b to 2d, in which a frequency inversion is indica-ted by f2 = 2B, f3 = 3B, f4 = 4B, and they are indicated as foldover products.
A foldover product can also be seen in ~igure 2e; this is not, however, disruptive since it is above the carrier frequency f9 = 9B.
Figure 3 shows a simple modulator with 2 multipliers, which manages simply with the two values V2 and 1, each with two signs and 0, and in which the scanning value x(kT) that is to be evaluated is passed by a selector switch, which is controlled by the drive logic of the analogue switch, to the corresponding one of the two inputs of a multiplier amplifier or to zero potential~
Figure 4 shows in detail the spectral relationships for continuous 4a -3~3~

signal processing.
The same spec~ral relationships may be achieved, in the frequency-range reproduced in Figure 4, even with a discontinuous system, i-f scanning frequency fs is suitably selectedO In this connection, fs mus~ be such that all of the harmonics of all square wave carriers fv for v = 1 to 5 must end up sym-metrical with half the scanning frequency. This ensures that no further unwan-ted lines are added, as a result of the periodicity of discontinuous signals, to the lines in the square wave carrier-spec~rum which are in any case present.
Conditions for this are:
fv m for m IN = 1, 2 O.O ~

In relation to half the scanning frequency, spectral lines then appear in the scanned square wave carriers at the following frequencies:
for m odd f25 + 2~ f and ,.
for mv even ~ 2~ o fv~
raspectively for ~ an elemen~ from the ser.ies of whole numbers Z = ~...-1, 0, +1 Ø~. The scanning ~requenc~ is therefore:
~s = 2nV . mv B = k . B.
Tabl~ 1 k fs/kHz nl n2 n3 n4 n5 S60 347,2 4 5 - 7 8 lZ60 781,2 5 6 7 - g 1008 624,96 4 7 8 9 1008 624,96 6 7 8 ~ -1680 1041,6 4 S 6 7 8 Table 1, above, shows cost-effective values for k, relevant scanning frequency fs, and for nv. Represented in the first four lines are solutions 3~

with no transposition of sub-bands 3, 4 and 5, which saves modulators.
In addition to the reduction in cost, the respective scanning frequency is lower.
According to the invention, scanning frequencies may be lowered s~ill further if sinusoidal carrier-waves, which can be assembled into corresponding series from only a few scanning values differing in amount, are used for the transposition. Here again, relationships ~4) and ~5) appl~. The following appl~ to t~e scanning frequenc~:
fs = KGv ~1. nv ~ i} . 1 ~ v = 1, O.. 5 or fs = ---r-~i k IN.

Because of the scanning theorem, however, k/l > 11 must be maintained~
;ln order to avoid spectrum-disturbing overlapping. Table ~ below shows pos-sible carrier-frequencies, in rela~ion to scanning frequency fs, as a f~lction of n, and for favourable values of i and 1, for the values 1/1 = 15, 20, 24, 39 and 36, in five corresponding columns.
Table ~
n fv/fs fv/fsfv~fs fv/fsfv~fs = 1 n - i 0~ 1/5 1/6 06 3/10 1/~ 1/5 1/6 ~6 E;3~

n fv/fs . fv/fs fv/fs ~v/fs f /fs = 1 O nk ~ i .. . . .. .

k/l 15 20 24 30 36 It may be gathered from ~able fY that if k/l = 15 with 1 = 2 and i = 1 only four of the permissible carrier-frequencies arise. Similarly, the last result shown for k/l = 36 with i = 0 is useless, since a maximum cf only four of the permissible carrier-re~uencies end up in an~ desired re~uency intervals of width lOB, so that at the most four sub~bands can be separa~ed ~ith equal fil-ters. Spectra after requençy-transposi~ion are glven in ~igure 2 for ~/1 = 24.
I:n the case of. values k/l = 24 and 307 ~avourable data for an arrangemen~ are listed in Table Z ~belo~3 ~r the carrier~requencies, again relative to t~e scanning frequenc~) for indiv.idual sub-bands, as is the mean f.requency of the band-pass filterO
Table Z
Sub-band fv/fs fv/fs fv~fS v/
. .
lf~ l/6 113, 114 l/5 3/lo 2 1/8 3/~ , 3/lQ l/3, 1/5 3 l/3 l/6 1lr3 l/6 4 3~8 l/8 l/lo 2/5 0, 5/12 1/2, 1/12 2/5 1/l~

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Sub-band fv/fS fv/fS f fv/fS

M 5 . B 7B 7B 8B
k/l 24 24 30 30 As regards discontinuous signal processing, the most cost-effective case arises with the choice of the follo~ing band-mean~frequency of the band-pass filter:
f~ = 5 B, since in this case a sub~band signal can be obtained with no frequency transpo-sition.
The modulators may be in the ~orm o a multiplying circui~ which multiplies the signal for fv/fs~ or whole number multiples thereof~ in ~ime wi~h the scanning frequency, according to Table 3 below, with the factors given therein:
Table 3 fv/fs 1/2 1/3 1/~ 1/5 1/6 1/8 1/10 1/12 0 ~`1 0 0 -1 l1actors -1 0.5 0 1 ~0.5 ~ 0.268 -0~618 ~~ +0O61~ +0.732

Claims (5)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A process for the transposition of equal width sub-bands of a signal frequency band by means of modulators, these being fol-lowed by equal band-pass filters as well as additional modulators, in which connection specific carrier frequencies (fl ...) are per-manently assigned to the modulators and between the band-pass filters and the subsequent modulators there is a coupling matrix (scrambling matrix) that is controllable by a random generator, said coupling matrix being intended for scrambling the sub-bands, characterized in that the carrier frequencies display the values f? = (n? - i/l) ? B
with ? = 1, 2 ... sub-bands of bandwidth B and i, and n?.SIGMA.?N = 1, 2 ... (elements .SIGMA. from the quantity of natural numbers N) as freely selectable parameters, in which connection i and n? can also assume the value of 0, and in that the modulators operate time-discretely and for the scanning frequency of the carrier oscillations the equation fS = 2m? f?.SIGMA.?, m?.SIGMA. ¦N = 1, 2 ...
applies.

2. A process according to claim 1, characterized in that the carriers are sinusoidal and -the scanning frequency is selected so that fS = KGV {1 n - i} B/I
with KGV equal to the least common multiple or to fS = k B/I with k.SIGMA.?N.

3. A process according to claim 1, whereby the signal frequ-ency band is the CCITT voice channel, characterized in that n = 5 sub-bands of bandwidth B = 620 kHz and in that the frequency transposition with square wave carriers takes place according to one of the following possibilities of Table 1:
Table 1

4. A process according to claim 1, characterized in that n = 5 sub-bands of bandwidth B - 620 Hz are formed, and in that the frequency transposition takes place according to one of the poss-ibilities of Table 3:
Table 3

5. A process according to claim 4, characterized in that the modulators consist of a multiplier circuit which multiplies the signal, for f?/fS or integral multipliers of this in time with the scanning frequency fS, in accordance with the following Table 4, with the factors cited therein:
Table 4