EP0367897A2 - Method for the simultaneous transmission of digital data signals in band-limited transmission channels - Google Patents

Abstract

The invention relates to a method for the simultaneous transmission of digital data in band-limited transmission channels. In the method, the data signals are rounded off before the transmission in order to achieve an adequate minimum signal/noise ratio between data signals and the other signals to be transmitted. The amplitude of the curve shape for rounded data signals according to the invention only assume values different from zero in an inner region of about 75 % of twice the bit width of one single bit in the time domain and are otherwise zero. The time function f(t) of the rounded-off data signal sequences no longer exhibit any saddles, the number of extremes of the first time derivation f'(t) is lower than the number of extremes of the first time derivation F'(t) of a time function F(t) which is rounded comparably but in accordance with EBU regulation. A preferred application of the invention is AM-RDS. The advantages of the method according to the invention consist, above all, in the minimum PM -> AM conversion and in the optimum eye pattern. <IMAGE>

Description

The invention relates to a method according to the preamble of claim 1.

Methods of this type are used for the additional transmission of digital data signals via an already existing band-limited transmission channel, which is mainly used for the transmission of other signals.

If additional transmission channels are used by data signals (e.g. radio data signals in radio), the program (main user) must not be disturbed by the data (co-users).

For this reason, the data signals must be rounded in such a way that they meet this condition. In most cases, this is done by severely limiting the spectrum of the data signals, e.g. with the FM radio broadcast radio data system ("FM-RDS") or with data transmission via a narrowband data channel, for example the sound channel from the studio (in the broadcasting house) to the transmitter. In other cases, the rounding must take place in such a way that the first time derivative of the rounded curve shape has a minimum of vibrations (extrema), e.g. with the AM radio broadcast radio data system ("AM-RDS").

From "Tech. 3244-E Specifications of the Radio Data System RDS for VHF / FM Sound Broadcasting" (Brussels, 1984), pages 5 to 10 of the European Broadcasting Union (EBU) is a temporal curve shape for a rounded data signal, the EBU -Curve shape, known, which was originally determined by the EBU only binding for VHF RDS and whose temporal course and spectral distribution in FIG. 1, which, however, only fulfills the condition of a hard band limitation, but not the condition of a sufficiently small number of oscillations in the first time derivative of the time function of the data signal sequence and therefore e.g. can only be used to a limited extent for AM-RDS.

The field of application for such waveforms is in principle the entire field of data transmission, although due to the non-linear properties of some systems (e.g. traveling wave amplifier in satellite radio) the time profiles of the data are distorted, which is why an exact shaping is often dispensed with.

According to the prior art, the rounding for slow data transmission can be optimally implemented with the aid of digital methods. Up to now, analog rounding has been common for fast data transmission, which therefore only approximates the theoretical values.

The EBU waveform designed as a biphase signal in FIG. 1a is symmetrical of origin and for T → ± ∞ oscillator very quickly approaches zero, where T is the width (duration) of the unrounded data signal (single bit). With this curve shape, amplitudes that differ substantially from zero can only be found in the range of ± 2T.

The associated spectral distribution in FIG. 1b is also symmetrical of origin and has spectral components that differ substantially from zero only within a range defined by the cut-off frequencies + Fg and -Fg, which represents the bandwidth of this EBU curve shape and which according to the above-mentioned. EBU regulation depends on the data or band rate. For biphase waveforms e.g. there is a range of ± 2 ˙ band rate. With a band rate of 1.2 kBd for FM RDS, for example, the bandwidth ± Fg is thus ± 2.4 kHz. In the case of AM applications, on the other hand, the band rate is ≦ 200 Bd, which results in a cut-off frequency Fg ≦ 400 Hz for biphase signal shapes in accordance with this EBU regulation.

To discuss the properties of the EBU curve shape that are of interest here, it is sufficient to consider the unmodulated data signal (baseband signal).

In FIG. 2a shows a typical chronological sequence of unrounded digital data signals in the NRZ (No Return to Zero) format.

With the according to the above EBU regulation made rounding (see FIG. 1) of the data signals results in the in FIG. 2b shown time function F (t) of the sequence of the now rounded data signals of FIG. 2a in biphase format. The corresponding eye pattern for this EBU waveform is shown in FIG. 6a.

Characteristic for this according to the above The time function F (t) formed by the EBU regulation are the saddles S in the area of the extremes (minima and maxima) of the function, which are also known in specialist circles under the name "dog bones".

In contrast to data transmission in the UWK-RDS, the digital data signals in the AM area, that is to say in the AM-RDS, are transmitted as phase modulation (PM) of the carrier. There is no EBU guideline for this service yet, but the one shown in FIG. 1 EBU waveform shown in principle can also be used in this area for rounding digital data signals, but this does not lead to an optimal solution.

Since the phase modulation (PM) for the data and the amplitude modulation (AM) for the message are orthogonal to one another, the two modulations in principle do not interfere with one another and can therefore be separated again at the receiving end.

In practice, however, there is mutual crosstalk from the data to the message and vice versa. Since, for reasons of compatibility, the interference caused by the data must not exceed a tolerance limit (e.g. signal-to-noise ratio> 40 dB), it is necessary to round the signal shape for the data and to limit the data speed and phase shift. Given the tolerance limit for the degree of compatibility, the aim is to round the transmitted data in such a way that the data speed and the data hub become as large as possible. (The data speed can then be increased if the phase shift is reduced, or vice versa. The delimitation of these two parameters from one another must be done on the basis of the probability of data errors.)

The size of the phase shift influences the immunity to interference of the data transmission. For a given bit error rate (e.g. BER = .0001) a phase shift of approx. ± 15 degrees is required. (This value depends on the interference phase shift of the transmitter and the receiver and is therefore subject to technical change.)

Assuming the value of the phase shift as fixed, the shape of the rounded data has a direct effect on the data speed (due to the compatibility) that can be achieved.

The program interferes with the data due to a conversion of the PM into an AM. Exactly expressed, the frequency modulation (FM) that is always associated with the PM is converted into an AM. This conversion of the FM into AM takes place in particular on asymmetrical edges of the intermediate frequency (IF) filter.

This illustrates FIG. 3, which shows the transmission curve U (f) of a typical IF filter as a function of the frequency f and which has an asymmetrical flank towards higher frequencies. This edge makes it symmetrical about a center frequency f frequency modulation with a frequency deviation Δf into asymmetrical fluctuations ΔU of the amplitude of the IF signal U around the center frequency f belonging amplitude value U implemented what can interfere with the reception of the main program.

The selected phase deviation Δφ and thus also the associated frequency deviation Δf and the shape of the rounded data signal are included in the magnitude of this disturbance.

Phase and frequency modulation are linked as follows:
If phase modulation includes a (data) signal s (t), frequency modulation includes a signal ds (t) / dt, i.e. the time derivative of the (data) signal s (t).

In FIG. 5a is again the one shown in FIG. 2b already shown and formed according to the above EBU regulation F (t) together with their first time derivative F '(t). Due to the sags S in F (t), the derivative F '(t) has a comparatively high number of vibrations or extremes, which, as practical tests have shown, a given certain propagation ranges conditions above the tolerance limit and thus audible "data hum" can result.

The object of the invention is to provide a method of the type mentioned, in which the additional data signals to be transmitted are rounded so that they also under extreme propagation conditions in data transmission systems that have a minimum of vibrations or extremes in the first derivative of the time function f (t) require the data signal sequence to be transmitted, such as AM-RDS, can be used in compliance with the specified minimum signal-to-noise ratio.

The achievement of the object is described in claim 1. The remaining two claims contain an advantageous embodiment and a preferred application of the invention.

The solution according to the invention provides temporal waveforms for rounded data signals, in which the minima of the time function f (t) of the sequence of the rounded data signals are only in the lower part of the permissible total amplitude range and the maxima accordingly only in the upper part, both of which the lower and the upper sub-area each make up about 25% of the total amplitude range. Furthermore, in the curve shapes according to the invention, the number of extremes (minima and maxima) of this time function f (t), on the one hand, and their first time derivative f '(t), on the other hand, is each less than or at most equal to the number of extremes (minima and maxima) the basis of the EBU waveform (see FIG. 1) for the same sequence of data signals Time function F (t) on the one hand and its first time derivative F ′ (t) on the other. Finally, the amplitudes of the waveform according to the invention, which are symmetrical of origin both in the time domain and in the spectral region, are different from zero (and otherwise zero) only in an inner partial region ΔTi (with the exception of the origin lying in the middle of the inner partial region) of the double bit width ± T of the individual rounded data signal ).

In an advantageous embodiment, this inner partial area ΔTi is advantageously approximately 60-90%, preferably approximately 70-80%, in particular approximately 75%, of twice the bit width ± T, the temporal curve shape advantageously consisting of three sections, each with a sinusoidal transition between directly adjacent sections consists.

The method according to the invention can be used particularly advantageously in data transmission systems which require a minimum of vibrations as extremes in the first time derivative of the time function of the rounded data signal sequence, such as e.g. the AM-RDS.

In the following the invention with reference to FIG. 4 to 6 explained in more detail.

The FIG. 4 shows, on the same scale as FIG. 1, the time function (a) and the spectral distribution (b) of a particularly advantageous waveform according to the invention (type C) for a single rounded data signal (single bit) in biphase format, which in the time domain (FIG. 4a) consists of three Parts with a sinusoidal transition between directly adjacent parts is composed.

Like the EBU curve shape, the type C waveform according to the invention is also symmetrical of origin both in the time domain and in the spectral distribution.

The amplitude of the curve shape C according to the invention assumes values other than zero in the time domain only in the inner partial region ΔTi (with the exception of the origin lying in the middle of ΔTi) and is otherwise zero (also in the two outer partial regions ΔTa).

In this exemplary embodiment, the inner partial region ΔTi is approximately 75% of twice the bit width (duration) ± T of a rounded data signal.

The bandwidth of the type C waveforms according to the invention is larger than that of the EBU waveform (FIG. 1) (which is why the waveform C according to the invention cannot be used for VHF RDS). The corresponding eye pattern for curve shape C is shown in FIG. 6b.

With the curve shape C according to the invention there are considerable improvements in the time function f (t) of the rounded data signal sequences and their first time derivatives f '(t), as shown in FIG. 5 clearly shows.

There are, on the one hand, the time function F (t) of the EBU shape (see FIG. 2b) and f (t) of the type C curve shape according to the invention, and on the other hand, the first time derivatives thereof F '(t) and f' (t), which result for the same (unrounded) NRZ data sequence (FIG. 2a), compared with one another.

As can be clearly seen, the time function f (t) of the data signal sequence rounded according to the invention no longer has sags in the area of the extrema (in contrast to the time function F (t) of the data signal sequence rounded according to the EBU regulation).

But the number of vibrations or extremes is also significantly smaller for f ′ (t) than for F ′ (t).

Since the time function f (t) of the data signal sequence rounded according to the invention has no sags and the first time derivative f ′ (t) of this function f (t) no longer has any unnecessary oscillations, the rounding according to the invention means that additional data signals can be transmitted even under extreme propagation conditions without " Data humming "is now possible.

This is particularly true for AM-RDS, since there the requirement for a minimum of vibrations in the first time derivative f ′ (t) is decisive in order to convert the FM signal into an AM, which is disruptive per se, to values below the tolerance limit press for audible data hum.

It goes without saying that the invention can be trained and developed with specialist knowledge and skills and can be adapted to the most varied of applications without this having to be explained in more detail here.

So the invention is not limited to that shown in FIG. 4 limited concrete embodiment explained in more detail; rather, other mathematical formulas can be found that give comparable curve shapes, i.e. which do not result in subsidence in the time functions f (t) and which have no unnecessary overshoots in their first time derivative f ′ (t).

Furthermore, the invention is not limited to the biphase format, but can also be used directly in the NRZ format (ie without converting the unrounded NRZ data sequence into a rounded biphase data sequence). This is particularly important for AM-RDS.

Finally, the application of the invention is not limited to AM-RDS, but also extends to similar data transmission systems in which the requirement for hard band limitation takes a back seat to the requirement for a minimum number of overshoots in the first time derivative f '(t ) the time function f (t) for rounded data signal sequences.

Finally, two essential advantages of the method according to the invention are to be emphasized, namely that a minimal PM → AM conversion and an optimal eye pattern are achieved.

Claims (3)

1. A method for the additional transmission of digital data signals via a band-limited transmission channel also used for the transmission of other signals, in which method to achieve a predetermined minimum signal-to-noise ratio between the digital data signals and the other signals, the digital data signals before transmission in their temporal curve shape are rounded, the time function f (t) of the sequence of the rounded data signals being defined by an overall amplitude range and the double bit width ± T of a single rounded data signal (single bit) being divisible into two outer partial regions ΔTa and an inner partial region ΔTi and the temporal curve shape and the associated spectral distribution are each symmetrical of origin,
characterized by
- That the minima of the time function f (t) of the sequence of the rounded data signals (C) are only in the lower sub-area comprising approximately 25% of the total amplitude range and the maxima also only in the upper sub-area also comprising approximately 25% of the total amplitude range;
- That the number of extremes (minima and maxima) of this time function f (t) on the one hand and its first time derivative f '(t) on the other hand is smaller or at most equal to the number of extremes (minima and maxima) based on one in "Tech. 3244-E Specifications of the Radio Data System RDS for VHF / FM Sound Broadcasting" (Brussels, 1984), pages 5 to 10 EBU curve shape defined by the European Broadcasting Union (EBU) for rounded data signals for the same sequence of digital data signals formed time function F (t) on the one hand and their first time derivative F '(t) on the other hand;
- That the amplitude of the temporal curve shape (C) assumes values different from zero only in the entire inner partial region ΔTi with the exception of the origin lying in the middle of the inner partial region ΔTi and is otherwise zero. 2. The method according to claim 1, characterized in that the inner portion ΔTi is about 60-90%, preferably about 70-80%, in particular about 75%, of the double bit width ± T and that the temporal curve shape (C) of the rounded data signal (Single bit) consists of three sections, each with a sinusoidal transition between directly adjacent sections. 3. The method according to any one of claims 1 or 2, characterized by the use for the additional transmission of digital data signals via an AM radio broadcast radio data system (AM-RDS).

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