AM Signal Research Articles

Photoconducting cells in the production of AM signals

Audio signals can be used to modulate the intensity of lightemitted by an LED. When this light falls on a photoconducting cell, thechange in resistance follow the variations of the audio signal. Thevariation in this resistance can be employed to control the gain of anamplifier. This article describes an IC circuit producing low-levelgeneration of AM signals using a conventional inverting amplifierconfiguration where the input signal is the carrier wave and a LDRcontrols the gain.

Optical chaotic AM demodulation by asymptotic synchronization

Synchronization between two identical chaotic systems (drive and response) with different initial conditions can be achieved provided that the conditional Lyapunov exponents are all negative. Adding an AM signal to the drive system, the two systems become asymptotically synchronized. The AM signal can be recovered by utilizing the property of the asymptotic synchronization. Chaotic AM noise is present as a direct result of the chaotic AM demodulation. Signal-to-chaotic-AM-noise ratio is calculated as a function of the signal amplitude.

Study of BER of 64-QAM signal and OMI-window of feasible operation in analog/digital hybrid SCM transmission systems

In an analog/digital hybrid subcarrier multiplexed (SCM) transmission, carriers have generally been substituted for transmission signals such as AM vestigal sideband (AM-VSB) AM signals and M-QAM signals to evaluate transmission quality. In practical hybrid SCM, however, carriers are modulated by video signals or digital data, and the amplitude of a multiplexed signal composed of these modulation signals is more compressed than that of the carriers. This causes a decrease in the frequency of clipping of the multiplexed signal at the laser threshold. Consequently, the BER of the M-QAM signal in a practical hybrid SCM is lower than that of the experimental results for the same optical modulation index (OMI). However, it is difficult to prepare many practical modulation signals for experiments in a laboratory. Therefore, there is demand for a bit error rate (BER) analysis method for a multiplexed signal that includes the modulation signals needed to sufficiently evaluate the BER and determine the optimum OMI in a practical hybrid SCM. In this paper, we describe such a BER analysis method that can effectively estimate the BER in a practical hybrid SCM. In practical systems, the BER was greatly improved over the BER of a multiplexed signal of carriers. Furthermore, BER degradations due to clipping can be neglected for the AM signals in setting a practical OMI range. We used this analysis method to study the effective OMI range of AM and M-QAM signals. By assuming modulation signals, the OMI range is enlarged and, significantly, the OMI of an AM signal becomes suitable for setting practical values in AM-SCM transmission. This OMI range is more practical than those of former studies.

A voice band Hilbert transformer using switched capacitor technology

A voice band (300 Hz to 3000 Hz) Hilbert transformer used for the modulation and demodulation of SSB waves and synchronous detection of AM signals by the phase synthesis method is realized by cascade connection of linear all-pass filters. In order to reduce the total normalized capacitance and the number of analog switches, a novel all-pass filter using switched capacitors (SCAPF) is proposed. Further, by using the multifrequency sampling technique, the capacitance ratio can be kept below 28. A sensitivity analysis of the phase difference on the capacitance is carried out. It is found by simulation that the phase difference is within 90 ± 0.75° if a 0.1% variation of the capacitance ratio is allowed. By means of experiments with individual components, stable operation of the SCAPF is confirmed. © 1999 Scripta Technica, Electron Comm Jpn Pt 2, 82(6): 8–16, 1999

Dichotic perception of AM and FM signals

The study deals with the binaurally perceived modulation depth (mp) of amplitude-modulated signals (Exp. I) and binaurally perceived frequency deviation (Δfp) of frequency-modulated signals (Exp. II), for their diotic and dichotic perception. Various combinations of modulation depth (m) and frequency deviation (Δf) at the left and right ear were used. It was found that when the interaural difference in modulation depth (Δm=ml−mr) or interaural difference in frequency deviation (ΔΔf=Δfl−Δfr) were equal to zero (i.e., diotic perception), the binaurally perceived modulation depth or binaurally perceived frequency deviation were the same as those in monaural perception. For sufficiently small Δm or ΔΔf, the perceived modulation depth, or frequency deviation, approximated the mean of the magnitudes presented to the left and the right ear. However, for large interaural differences in Δm or ΔΔf, the binaurally perceived quantities were lower than their arithmetic means, and showed nonlinear trends. Comparison of the perceived modulation depths determined in the experiment and those calculated under the assumption of the linear summation of AM signals from the left and the right ear are also discussed.

AM and FM calcium signals

AM and FM calcium signals

Carrier wave frequency multiplication of AM signals

Until now carrier wave frequency multiplication has been applied in CW, PM and FM techniques. However, it has never been considered as applicable to AM signals. This is due to unsupported opinions that it would distort the envelope of AM signals. Theoretical considerations were carried out with the use of the double Fourier series expansion. The results have shown that it is enough to have an uninterrupted flow of the envelope through the amplifier to have a nondistorted envelope in both amplification and frequency multiplication. The results were verified experimentally and by observations of BC and TV stations at their harmonic frequencies. The conclusion has been made on AM signals having two or more frequency fringes, e.g.: single tone DSB, two tone SSB.

Delta-sigma oscillators: versatile building blocks

Delta–sigma modulators have long been used in signal-processing applications to simplify arithmetic operations. Their ability to trade bandwidth for quantization levels has been exploited to reduce hardware requirements and speed up computations. This paper will introduce a class of circuits called delta–sigma oscillators which use this property to efficiently generate spectrally pure analogue signals. Subsequently we will show how this approach can be extended to generate various multitone signals such as AM and FM signals. Delta–sigma modulators with various noise-shaping properties are at the core of these new designs. Owing to their small-silicon-area requirements and digital implementation, delta–sigma oscillators have been identified as an important building block for built-in self-test schemes for mixed-signal circuits and systems. They are also envisioned to be useful building blocks for various control and communication system applications. © 1997 by John Wiley & Sons, Ltd.

Apparatus and method for reducing acoustic feedback

The AM signal (12a) from a microphone (11) is converted (13) into an inaudible, FM signal (13a) which is combined (14) with the AM signal to form a composite signal (14a). The composite signal is amplified (16) and converted into sound waves by a speaker (17). Any sound returning to the microphone from the speaker has an FM component and the FM component is demodulated (26) to reconstruct the original AM sound. The reconstructed AM signal is subtracted (29) from the signal from the microphone, thereby removing the echo and cancelling feedback.

Limits on discrete modulated signals

We develop theorems of a general nature that apply to the analysis of AM-FM signals of the form a(m)exp [j/spl phi/(m)] or a(m) cos [/spl phi/(m)] and to their behavior both in linear systems and in simple nonlinear systems comprised of products of linear elements. Such product-systems include interesting nonlinear demodulation operators, such as the Teager-Kaiser (1990) operator. Expressions for the approximate system responses to AM-FM signals are derived by making an analogy to the eigenfunction interpretation of sinusoids in linear systems; for the case of sinusoidal signals, the approximations are exact. These expressions are collectively called quasieigenfunction approximations (QEAs). For nonsinusoidal AM-FM signals, the approximations have errors that are tightly bounded by functionals that express the smoothness of the AM and FM information signals and the durations of the involved system impulse responses. The bounds are independent of the bandwidths of the AM and FM functions. Two general applications are considered. First, the approximations are found to be useful for analyzing discrete-time nonlinear energy operators, including the Teager-Kaiser operator. Next, the approximation theorems lead to the selection of an optimal class of bandpass filters for use in a discrete multiband AM-FM demodulation system. The filter class selected is optimal in the sense of achieving the lower bound of a novel discrete uncertainty principle.

Dichotic perception of modulation depth of AM signals

The aim of this study was to evaluate the resultant value of the modulation depth perceived binaurally when sinusoidally amplitude-modulated tones, with different modulation depths, were presented to the left and right ear. Different values of the modulation index (ranging from 0% to 100%), modulation frequency (4, 64, 128 Hz), and carrier frequency (250, 1000, 4000 Hz) were used in the experiment. The data show that for low values of the modulation indexes, the dichotic perception of modulation depth approximates the arithmetic mean of the indexes presented to the left and right ear. However, this is not the case for higher values of the modulation index for which the perceived modulation depth is always lower than the mean of the indexes at both ears. Additionally, it was stated that perceived modulation depth does not depend on the rate of modulation for relatively small interaural differences in modulation indexes. In turn, for larger interaural differences the perceived modulation depth tends to decrease with increasing modulation rate. The model of binaural perception of AM tones is considered. a)Permanent address: Institute of Acoustics Adam Mickiewicz University, Poznan ul. Matejki 48/49, Poland.

Complex AM signal model for non-stationary signals

A novel signal model consisting of a weighted sum of complex amplitude modulated signals is proposed as a suitable representation for non-stationary signals like speech. The estimation of model parameters is carried out by utilizing the accumulated autocorrelation functions of the modelled signal. The developed model is first fitted on noise-corrupted synthesized data, and then on sampled voiced speech data. The study demonstrates the suitability of the model.

Multichannel 64-QAM transmission in AM/digital and all-digital hybrid-fiber-coax based video dial-tone systems

A hybrid-fiber-coax (HFC) system is operated with a combination of 63 AM and 21 64-QAM subcarriers as well as with the 21 64-QAM subcarriers alone. The 64-QAM subcarriers are 6-MHz wide, decorrelated, and of the type intended for compressed digital video. The AM signal levels (over both the optical and coaxial links) are set to produce standard carrier-to-noise ratios (CNRs), composite second order distortion (CSO), and composite triple beat (CTB). The optical modulation depth (OMD) is independently varied and transmission tests are carried out over the optical link alone, as well as over the entire system (including two coaxial amplifiers, or actives). The relative degradation of the system due to the optical link versus the actives is investigated. For an OMD of 24% or less and with the raw bit-error ratio (BER) set to 10/sup -5/ (as anticipated for use with forward error correction (FEC) systems), the performance is limited by the actives. There is an approximate 3.5 dB penalty in the necessary CNR for 64-QAM reception, due to the presence of the AM signals. On the other hand, when the system is operated with all-digital subcarriers, the optical modulation depth may be increased from 24% to 37% with no penalty in the required digital CNR.

Efficient demodulator for bandpass sampled AM signals

A simple nonlinear (quadratic) filter is shown to demodulate bandpass sampled AM signals efficiently. The filter is based upon a discrete version of the recently introduced Teager-Kaiser energy operator, but also closely resembles a complex digital sampling demodulator. Such a filter can also be implemented in analogue circuitry.

Development of 128 optical distribution system of 150 chs am/qam hybrid signals

AbstractAn optical distribution system for providing CATV services to approximately 30,000 subscribers using a fiber‐to‐the‐curb (FTTC) system has been developed. This system distributes the optical signal to 128 paths for 90 carriers with a hybrid 150 channel signal which accommodates 60 AM signals and 90 16 QAM signals to provide both conventional service and future services. The development of low distortion and low chirp MQW 1.55 mm DFB‐LD is discussed and the zero distortion conditions for optical fiber amplifiers are verified in the investigation of the main parameters in the design of the system. To satisfy the system specifications, the signal wavelength is chosen to be 1561 nm, the input optical power is set to 6 dBm, and a two‐stage optical fiber amplifier structure is used. The results of transmission tests are shown to verify the performance of the system. Lastly, the potential for expansion of the optical distribution system is discussed.

Preclipping experiment for subcarrier-multiplexedAM/64QAM optical transmission

Bit-error-rate degradation of the 64QAM signal due to clipping distortion caused by overmodulation at the laser diode is compensated for by preclipping for subcarrier-multiplexed AM/64QAM optical transmission. The worst composite distortions of CSO < –60 dBc and CTB < –65 dBc have been achieved in the 40-channel AM signal band. It has also been confirmed that preclipping is more effective than Reed-Solomon (204, 188) error correction against clipping distortion.

Asymptotic clipping noise distribution and its impact on M-ary QAM transmission over optical fiber

Performance analysis for a hybrid subcarrier-multiplexed (SCM) AM/QAM transmission over an optical fiber is presented. It is shown that the bit-error-rate (BER) of M-ary QAM in such hybrid systems can be significantly affected due to occasional laser of the SCM signal. Here we analytically determine the asymptotic distribution of the clipping noise by modeling it as a Poisson impulse train. The BER of M-ary QAM is then evaluated in terms of optical modulation indices of M-ary QAM and AM signals which in turn specify signal-to-noise ratio (SNR), impulsive index (clipping index) of the clipping noise, and power ratio of the Gaussian noise to the clipping noise. Numerical examples are given and compared with experimental data with reasonably good agreement for small SNR's. The results have application for estimating the BER's of digital signals for SCM analog/digital transmission over an optical fiber and for employing appropriate error correction codes and/or optimum receiver design for such environments. >

Roughness and the critical bandwidth at low frequency

A plausible model represents the sensation of roughness as the product of three factors, a speed factor that increases with increasing modulation rate, a temporal modulation transfer function (TMTF), and a spectral window that is determined by auditory filter widths. At high signal frequency, where critical bands are broad, the TMTF is the controlling factor. At low signal frequency the auditory filters dominate. Therefore, roughness experiments can serve to measure the critical bandwidth at low frequency. To implement this idea, listeners adjusted both the modulation rate for AM signals and the beat rate of beating sine pairs in order to achieve maximum roughness. The center frequency varied from 2000 to 70 Hz, and SPLs were 60 and 80 dB. The data could be fitted with a model in which roughness is summed over all auditory filters. The data require the critical bandwidth parameter to continue to decrease with decreasing frequency below 500 Hz so that it becomes considerably narrower than critical bands fr...

Identification algorithm of upper sideband and lower sideband SSB signals

An intelligent algorithm capable of recognising AM, FM, SSB and continuous wave (CW) signals has been recently described. Such an algorithm is necessary in intelligent receivers that can detect unknown signals without any user intervention. A novel technique of identifying the upper sideband and lower sideband cases of SSB is presented here. This separation algorithm is computationally simple and is particularly suitable for speech modulating signals due to their typical spectral shape. The simulation results demonstrate the effectiveness of this separation technique, and it is found that successful separation can be achieved even for very low SNR values.

Signal distortion and noise in AM-SCM transmission systems employing the feedforward linearized MQW-EA external modulator

In subcarrier multiplexing (SCM) optical video distribution systems, the nonlinear signal distortion generated by the combined action of laser chirp and fiber dispersion limits the transmission distance. This paper for the first time shows that low-chirp MQW (multiquantum well)-EA (electroabsorption) external modulators are applicable to AM-SCM transmission systems. The feedforward linearization technique is used to compensate the signal distortion due to the nonlinearity in the modulator's L-V (light power versus voltage) characteristic. The effectiveness of this type of modulator for suppressing dispersion-induced distortion is clarified experimentally. A feedforward linearized transmitter with composite second order (CSO) intermodulation distortion 50.5 dB is successfully constructed. It is shown that the transmitter can achieve a CSO 49.5 dB for a 32 channel AM signal and 10 km transmission. Furthermore, estimations of the signal distortion cancellation and the noise characteristic achieved with feedforward circuit modeling are shown. The results obtained here emphasize that MQW-EA external modulators are applicable to AM-SCM transmission systems. >