The signal breaking in on frequency 6 1 5 5 originates in Central Europe from the European service of „Österreichischer Rundfunk“.
It can sound almost as good as the first channel of your national FM programme, or it can be croaky or muffled and hardly identifiable. In these cases, the operating mode is probably not correctly set.
AM (A3) (amplitude modulation): Broadcasting radio stations transmit a carrier signal and two sidebands with identical sound information, the tuning is quite uncritical.
USB / LSB (Upper side band / Lower side band): Amateur radio and commercial voice communication is usually broadcast in single sideband mode (SSB). ins this mode, only one sideband with the voice information is transmitted and all available high-frequency power is used economically to carry the modulation. In the receiver, a reconstructed carrier signal is mixed with the sideband signal. Since reception is not depending on the carrier signal „getting through“, in many cases, very weak stations can still be heard. In this mode, the tuning with the locally reconstructed carrier signal must be very accurate, if the carrier is inserted a few Hertz too high or too low, the speaker gets a bass or Mickey Mouse voice and the music sounds completely strange with the wrong pitches. If the tuning steps are too large, you will hear the so-called musical scale effect when you tune the receiver across a station's signal.
CW (A1): „Morse code signals“ or so called „continuous wave signals“ , where only the carrier signal is switched on and off, are also only audible, when a carrier signal is inserted in the receiver. The frequency fine tuning changes the pitch, many sets have a knob to control the pitch of the BFO (beat frequency oscillator). Some receivers have a special position on the mode switch for receiving CW signals, with others, SSB must be selected.
F3|FM (F3)]: frequency modulation is used in the highest range of the shortwave spectrum, for example by CB radio operators, but frequency modulation is also standard in the VHF band. Professional communication receivers only offer reception of frequency-modulated shortwave stations in the so called „narrow band FM mode“. In other multiband or „all wave“ receivers, the VHF frequency range of 87.5 - 108 MHz of the FM broadcast band is covered, so the receiver can be used to listen to domestic radio programmes.
RTTY / FAX: with certain receivers, a SSB mode with a special filter bandwidth and a slight frequency offset is available for radio teletype or FSK operating modes (F1), but radio teletype signal (RTTY) can also be heard with receivers capable for single sideband mode.
8: The operation modes are switched in different ways. In many sets, a rotary switch is used, other have a selection of pushbuttons for AM, USB, LSB, CW, FM. The operating mode is often indicated with LEDs or letters on the receivers display. The bad habit of coupling certain operating modes with automatic switching of the IF filter bandwidth was intended to make operation easier for inexperienced listeners, but receivers with independently switchable operation modes and IF filter bandwidths are to be preferred.
9: Pressing a SYNC or ECSS key activates the synchronous detector. Only the sideband information of a received shortwave signal is demodulated; the carrier signal, which is more subject to fading and propagation disturbances, in this mode is not required for demodulation. Instead, a substitute carrier signal is generated in the receiver itself. A special circuit automatically moves the internally generated carrier to the correct distance from the signal and locks, so that no whistling, bassy or Mickey Mouse voices occur. The engagement of this circuit, usually accompanied by a short whistle, is usually signalled by an LED indicator.
10: Intermediate frequency (IF) or bandwidth filters can be selected out of a choice of two ore more filters with a NARROW / WIDE - switch or a rotary switch, to change the IF bandwidths. After an incoming radio signal is mixed to generate an intermediate frequency in a superhet receiver, the IF signal has to pass a more or less narrow filter that will only allow frequencies very near the intermediate frequency to pass. This will make up the selectivity of a receiver, the capability to select one out of a few signals on very nearby frequencies in a shortwave band.
Using a narrow IF-filter allows to eliminate unwanted signals from interfering signals on a nearby frequency more effectively. But when a narrow IF bandwidth / filter is in use, audio becomes bassy or dull.
A second important technical parameter of an IF filter is it's skirt selectivity: With very steep shoulders or „skirts“, interfering signals can be cut off easier, wile the information - bearing parts of the audio signal still can pass.
For standard broadcast signal, i.e. AM reception, an IF filter with a bandwidth of 4 - 6 kHz will do a fine job, for SSB reception, you need a 1.8 - 3 kHz filter, which will be wide enough to let the single sideband pass through. For CW reception of Morse code signals, sometimes filters with a bandwidth of 250 - 500 Hz are provided, these allow to virtually cut the signals from one station out of a mix of several interfering transmitters. For audio transmissions, these filters are not suited, you won't understand the spoken word nor music.
Some sets like the NRD-535 provide a variable bandwidth (10a), this technology is nowadays replaced by digital processing of the HF signal, with a virtually unlimited number of IF filter bandwidths using DSP technology.
With correct settings, the „Österreichischer Rundfunk“ signal should be audible in good quality like a local AM station.