Telephonic communications – Substation or terminal circuitry – For loudspeaking terminal
Patent
1987-10-15
1989-12-05
Ng, Jin F.
Telephonic communications
Substation or terminal circuitry
For loudspeaking terminal
379388, 381 98, H04M 160
Patent
active
048857726
DESCRIPTION:
BRIEF SUMMARY
TECHNICAL FIELD
The invention relates to an apparatus for obtaining a high sound level and good sound reproduction from a loudspeaking telephone. The apparatus is particularly suited to a telephone powered from the line, in which the power supply is limited.
BACKGROUND ART
The loudspeaker amplifier in a line-powered loudspeaking telephone is easily excited to maximum level due to only limited power being available from the telephone network. This causes distortion, which is often already noticeable at normal speech levels and which can be heavy for high speech levels.
Simple and cheap loudspeaker elements usually have small diaphragm masses and small magnet systems, which give low mechanical attenuation and thereby a high Q-value. The efficiency of such loudspeaker elements is considerably higher for frequencies in the vicinity of the resonance frequency than for higher and lower frequencies. In the utilisation of such loudspeaker elements, a large part of the acoustic effect must therefore be attenuated with the aid of suitable absorbents in a sealed loudspeaker box so that the resulting efficiency will be relatively uniform. It will also be rather low, however. It is therefore generally the case in loudspeaking telephones to use expensive loudspeaker elements with relatively uniform efficiency. However, here also the elements must be mounted conventionally in a closed box with absorbents for attenuating resonances.
DISCLOSURE OF INVENTION
The object of the present invention is to provide an apparatus with which the power supplied to a loudspeaking telephone may be utilised effectively such as to give a high sound level and good sound reproduction. This is achieved by the apparatus including a loudspeaker element with a relatively high Q value, the resonance frequency of which is in the vicinity of the power maximum of a speech spectrum, and by utilising a loudspeaker amplifier with frequency-dependent gain, which compensates for the frequency-dependent efficiency of the loudspeaker element. With the aid of the apparatus in accordance with the invention a simple and cheap loudspeaker element can be utilised without being mounted in a closed box provided with attenuating absorbents. The apparatus also achieves that the sound level may be higher and distortion lower than in conventional apparatus having a uniform frequency response in the loudspeaker amplifier and uniform efficiency in the loudspeaker.
The characterizing features of the invention are disclosed in the claims.
BRIEF DESCRIPTION OF DRAWINGS
The invention will now be described in detail below with reference to the drawings, on which
FIG. 1 illustrates a tone curve for a loudspeaker with relatively uniform efficiency,
FIG. 2 illustrates a curve of the frequency spectrum of a conventional loudspeaker amplifier,
FIG. 3 illustrates a resulting tone curve for a loudspeaker with relatively uniform efficiency and a conventional loudspeaker amplifier,
FIG. 4 illustrates a tone curve for a loudspeaker element used in accordance with the invention,
FIG. 5 illustrates a curve of the frequency response of a loudspeaker amplifier used in accordance with the invention;
FIG. 6 illustrates a resulting tone curve for a loudspeaker element and a loudspeaker amplifier, which are used in accordance with the invention and
FIG. 7 illustrates the principal elements of the present invention.
BEST MODE FOR CARRYING OUT THE INVENTION
Referring first to FIG. 7, the principal elements of the present invention are a loudspeaker 13 as driven by an amplifier 11, both incorporated, for example, in a telephone 9. The amplifier is connected to the telephone line 15. Such a combination is, of course, known in the art, the present invention more particularly residing in the characteristics of the loudspeaker 13 and the amplifier 11, to be described presently.
An example of a tone curve is illustrated in FIG. 1, i.e. the acoustic power as a function of the tone frequency, for a loudspeaker with relatively uniform efficiency. According to the example, the efficiency in
REFERENCES:
patent: 4315109 (1982-02-01), Jacobson
patent: 4340778 (1982-07-01), Cowans et al.
patent: 4481662 (1984-11-01), Long et al.
patent: 4709391 (1987-11-01), Kaizer et al.
Ng Jin F.
Telefonaktiebolaget L M Ericsson
Vaas Randall S.
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