SHIP-BOARD MUSIC APPLICATONS
By: Don McClathcie
The US Navy and Vacation Cruise Lines have long supplied entertainment on-board ship for the passengers and crew. The easiest way to distribute this entertainment is on a single coaxial cable or cable system. These ship-board cable systems are designed exactly like the ones on land.
Most of the ships at sea already have coaxial cable to distribute television programming, usually from recorded sources. These coaxial cables can also be used to distribute Stereo Music programming in the 88-108 MHz band in between the standard TV channels. As many as 50 music channels can be multiplexed onto the TV distribution system using a channel spacing of 400 KHz. You can put as many as 100 channels on the system using a channel spacing of 200 KHz if the output power levels are carefully set to the same level.
Many TV Sets that are sold to the hotel industry have FM reception capability, and of course any standard FM radio receiver can be connected to the cable system to receive the music programming directly without the need for a TV set.
The system requires a program source, usually provided by multi-disc CD players and an FM Multiplex Modulator. The FM Multiplex Modulator converts the base-band left and right audio signals from the CD player into an FM Modulated Carrier set on a frequency between 88 and 108 MHz.
This FM signal is then combined with other signals on the cable system by daisy chaining the outputs of the Multiplex Modulators or by using an external RF combiner. The FM Multiplex Modulator should be able to handle all sources of audio including mono sources and if it has a built-in stereo synthesizer even mono audio sources will be distributed in stereo.
It is also useful for the Modulator output frequency to be field agile (adjustable) so the operator can change the RF channel if needed. Stereo signals can also be received “off the air” in a home port and placed on the ship-board entertainment system to provide local news and information. That home port station can then be recorded and played back in a loop until the ship returns to home port and has live reception again.
It is even possible to tune an FM Band Stereo Modulator to a frequency outside of the FM Band to distribute audio programming in the A1-A2 band adjacent to the FM band that would not ordinarily be accessible to other FM receivers. This would enable “soft security” channels that could be used to distribute audio to other specific locations on the ship for instruction and training purposes that could not be accessed by standard FM receivers connected to the TV cable system.
HOW DOES THE FM MODULATOR DO IT?
The Stereo Multiplex Modulator unit transmits two 15 KHz audio channels, a left and a right audio channel on a single radio frequency channel. To accomplish this, two audio channels are first Pre-emphasized and then put through a Matrix that creates a L+R (SUM) channel and a L-R (Difference) channel. The sum channel alone delivers mono audio to the FM receiver, this is the audio that you hear when your receiver switches to DX for distant stations, and the sum & difference channels are both used to deliver stereo to the FM stereo receiver.
The sum & difference audio channels from the matrix are then fed into a Four Quadrant Multiplier along with a pilot frequency of 19 KHz and a carrier frequency of 38 KHz. The pilot frequency is used by the FM receiver to lock onto the 38 KHz carrier to decode the difference information and is used to indicate the presence of the stereo signal. The difference information is modulated onto the 38 KHz carrier and converted into a double side band suppressed carrier (DSBSC) signal. The mono sum information is then added to the DSBSC signal and is then sent to the FM Modulator.
The wideband DSBSC multiplex signal is now ready to be frequency modulated onto the main FM Band carrier. In the modulator the multiplex signal causes the main carrier frequency to change or deviate and in so doing transmit the stereo programming on the FM Band frequency for use by the FM radio. If the Multiplex Modulator is also equipped with a stereo synthesis system, it is also possible to create a stereo signal from a monaural program source.
In a stereo receiver these two (L+R and L-R) channels are decoded using the pilot then re-combined to produce the original Left and Right audio signal and then De-emphasized to overcome noise in the transmission process.
Audio program distribution using FM Band modulators is a well proven, time tested way of providing a vast array of audio entertainment to passengers and crew aboard ships in port and at sea.