5.1 Channel FM Transmitter / Receiver System
Design Review Document
December99-05
15 April 1999
Senior Design
John Lamont
¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯ / ¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯Robert Frank / Kirk Chervenka
292-8247 / 296-0619
/
¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯ / ¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯
Joseph Spinks / David Berka
296-4192 / 268-0000
/
Abstract
The purpose of this project is to design and build a receiver transmitter pair to implement a new FM waveform allowing the broadcast of 5.1 channel surround sound. The 5.1 channel surround sound waveform will consist of the standard subwoofer, left, right, and center front channels, along with left and right rear channels. The new waveform and transmitter will be constructed such as to be backwards compatible with the currently implemented FM mono and stereo broadcast formats.
Table of Contents
Abstract......
Table of Figures......
Table of Tables......
List of Acronyms......
Background......
Design Objectives......
Waveform......
Transmission and Reception......
Proposed Technical Solution......
Milestones......
Proposed Budget......
Proposed Schedule......
Contacts......
Table of Figures
Figure 1: Basic Structural Components......
Figure 2: Current Encoding Method for FM broadcasts......
Figure 3: Proposed Encoding Method......
Table of Tables
Table 1: Modulation Equations......
1
List of Acronyms
AMAmplitude Modulation
CDCompact Disc
DSB-SCDouble Side Band Suppressed Carrier
DSPDigital Signal Processing
DTSDigital Theater Sound
DVDDigital Video Disc
FMFrequency Modulation
QAMQuadrature Amplitude Modulation
SDDSSony Dynamic Digital Sound
Background
The time for surround sound audio music has been a long time in coming. From its beginning the audio industry has been driven by the goal of realistic repeatable audio reproduction. The initial audio format used in both home and theater was monaural sound. The film industry led the movement away from monaural in the early 1950's with stereophonic sound. Stereophonic sound as it was called consisted of a minimum of four channels, including at least one speaker near the back of the theater known as the effects channel. The effects channel was the first step towards what is now known as surround sound. This channel was mainly used for dramatic effects and was initially of very low fidelity. As time went on the effects channel fidelity was increased and the channel was used to introduce new spatial sounds known as surround sounds. After the initial development of theater surround sound the main battle between film industry and audio equipment producers has been the production of a standardized storage and playback format. In recent years the theater format battle has waged between Dolby Digital, Digital Theater Sound (DTS), and Sony Dynamic Digital Sound (SDDS).
While surround sound has been a long accepted practice in theaters it is not until recently that it has found its way into the home entertainment environment. The stereo sound format is currently dominant in the consumer audio realm. Stereo was introduced into the home in 1958 with only two channels as compared to the minimum of four channels used in theater stereo systems. The reduction in channel number from a minimum of four to two was necessary due to the limited storage of the LP phonograph. A few years FM broadcasting followed suit, and introduced FM stereo sound. Stereo was widely adopted and supported by the public and has remained the leading if not only audio format for most home audio applications.
An attempt was made in the early 1970's to oust stereo from its lead position in the market through the introduction of the quadraphonic sound format. This format consisted of four channels encoded in the current media on the unused quadrature phase. The quadraphonic system failed due to the inability of the consumer to recognize any benefit in using the format, along with the fact that most recordings of the day were in the stereo format.
The rise of home theater systems, laser disks, and DVD technology has brought the once rejected home surround sound system into reality. The surround sound has mainly been applied to movie sound reproduction, but the important point is the consumer awareness that has been brought about. This raised level of consumer awareness along with the new media make this the perfect time for the introduction of surround sound formats into the audio music world.
Consumers already have adopted the 5.1 channel standard supported by Dolby Digital (AC-3), DTS, and SDDS for their home movie applications, and therefore already have the speaker and amplifier systems supportive of surround sound. These currently available consumer resources need to be tapped into by the audio music industry to bring about a surround sound music revolution. Currently the mainstay of the music industry is the Compact Disc (CD). The CD is an excellent format with very high 16 bit fidelity, but its one drawback is that of limited storage space. The CD faces much the same problem as the LP faced in 1958, its storage space limits it to stereo sound reproduction. However the new DVD media is beginning to take its place in the industry. This new media does have the required storage space to include information for the required 5.1 channel format. The consumer view of surround sound in music may be somewhat limited, and the price of DVD players, and high quality surround sound systems are quite high, producing a reluctance to invest in a system that may fail in the same manner as quadraphonics.
It is obvious that 5.1 Channel FM broadcasts must be used to encourage the production and purchase of surround sound music. The 5.1 channel FM broadcast system would take advantage of the home theater systems currently capable of processing and reproducing the surround sound format, and with slight modification of current automotive amplifier arrangements, make use of the high quality multi speaker systems installed in a majority of automobiles.
On the consumer end, reception of the 5.1 channel format would only require the purchase and installation of a special FM receiver, which would be capable of receiving 5.1 channel FM, FM stereo and mono, and AM signals. The receiver would not be much higher in cost than current receivers due to the ability to do all of the digital format decoding at the radio station. The receiver could easily be adopted for car or home applications.
The radio stations would not require a great investment or re-fitting to implement the new 5.1 channel standard. Since the waveform will be designed to be reverse compatible with current FM stereo and mono waveforms it will use the same FM modulator and transmitter. The station would only have to invest in a special base band mixer. The base band mixer operates at low frequency and power, and therefore would not be high in cost. The radio station would also be able to transmit all of their programming in the 5.1 channel format, while retaining their stereo listeners due to the stereo compatibility of the waveform. It can be seen that with little investment a radio station will be able to literally introduce another dimension to its broadcasts.
Design Objectives
This project shall be divided into two main sections: (1) waveform, and (2) transmission and reception.
Waveform
The waveform shall incorporate the 5.1 channel surround sound standard into the current FM stereo signal structure. The waveform may need to incorporate quadrature multiplexing along with single side band modulation techniques into the FM base band signal to introduce the additional channels with minimal FM stereo reception distortion.
Transmission and Reception
The transmitter and receiver designs will be dictated by the waveform design used. The transmitter and receiver may need to utilize DSP technology to encode and decode the complex modulation techniques proposed. The FM modulator and transmitter will have to stay in the standard format currently in use to minimize the cost of introducing the new technology. The main addition to the transmission process will be that of a special base band mixer responsible for inserting the additional information into the current FM base band format. The receiver will also have the same FM demodulator building block currently used, but will need to incorporate a new base band signal demodulator, with which to separate the 5.1 channels.
Proposed Technical Solution
Figure 1: Basic Structural Components
Figure 2: Current Encoding Method for FM broadcasts
Figure 3: Proposed Encoding Method
We plan on designing a modulation scheme that makes use of the audio bandwidth available in the current FM stereo format. Available for our use is 4kHz on either side of the 19kHz pilot, the frequencies between 53kHz and 72kHz in what is called the supersonic portion of the signal, and the quadrature or imaginary side of the 23kHz to 53kHz bandwidth. One proposed spectral structure is shown above in Figure 3. This picture is one of many possible modulation techniques that we are considering, though it is not necessarily the waveform that will be used.
The proposed design modulates the right and left rear channels (3 and 4 as seen in the figure) on either side of the 19kHz pilot utilizing single side band amplitude modulation. The front center channel (represented by a 2 in figure 3) is then modulated with 15kHz audio bandwidth on the upper side band of a 57kHz carrier. This will be performed using single side band modulation. The subwoofer channel is modulated on the same 57kHz carrier as the lower single side band signal.
As can be seen in Figure 3, single-sideband (SSB) modulation is the technique that we have utilized in the proposed spectral format. Single side band amplitude modulation, and quadrature amplitude modulation were the two techniques investigated, both of which double the information inserted into a given frequency bandwidth. The single side band technique was chosen over quadrature amplitude modulation (QAM), because of our ability to introduce it with more predictable separation from the current FM stereo signals. The advantage that SSB holds over QAM will now be further investigated.
Single side band or SSB signals rely on the use of what is known as the Hilbert transform. This transform has a singularity at the origin of the frequency axis and results in the positive frequencies to be multiplied by -j, and the negative frequencies to multiplied by j, j being the square root of -1. The transformed signal is then multiplied by j and is then either subtracted or added to the original signal. This produces a frequency response only on either the positive or negative frequency portion of the signal around its modulated frequency dependent on the addition or subtraction of the Hilbert transformed signal from the original signal. A frequency response isolated to the positive frequency components is called USSB, or upper single side band, while one isolated to the negative frequency components is called LSSB, or lower single side band. The advantage of SSB in general is the ability to modulate a signal and only utilize half of the bandwidth that the signal requires at base band, due to the redundancy of double side band signals.
Quadrature amplitude modulation utilizes it's bandwidth by modulating one signal with a carrier using double side band suppressed carrier modulation, and then modulates another signal with a carrier 90 degrees out of phase with the first carrier. This form of modulation in essence modulates one signal with a completely real signal while modulating the other with an imaginary signal. The advantage of QAM is the ability to use bandwidth in which a real signal is already double side band suppressed carrier modulated. The disadvantage of QAM is the fact that it is a double side band signal. The complex envelope representations of these forms of modulation are shown in the table that follows:
Table 1: Modulation Equations
Type of Modulation / Mapping Functions g(m) / Corresponding Quadrature Modulationx(t) {Real} y(t) {Imaginary}
Quadrature Amplitude Modulation / / /
Single Side Band Amplitude Modulation with Suppressed Carrier / / /
where is the Hilbert transform of .
The above table shows that given two real input signals the QAM method results in the ability to use a given frequency bandwidth twice over. However, given two complex input signals the respective output signals will be decoded improperly at the receiver as the mix of real and imaginary components of the two signals. This QAM complex input signal problem is a concern for our project even though the audio signals used are real. If a phase offset is introduced on our audio signals at any point through filtering, modulation, or anything else the QAM complex signal problem will crop up. This states that the transmission filtering and modulation would require perfect phase retention on the QAM transmitted signals, and the receiver also needs excellent phase performance. Therefore, due to the fact that the system needs to be backwards compatible the possibility of receiving a distorted quadrature rear audio signal component on the in-phase stereo right and left channels is unacceptable and QAM will not be used on currently occupied frequency bands of the FM stereo spectrum.
QAM also appears as the secondary choice for modulation in the vacant frequency bandwidth of the FM stereo spectrum. QAM requires a symmetrical frequency bandwidth, this is due to the fact that QAM is composed of two double side-band suppressed carrier signals modulated 90 degrees out of phase. Therefore, if it is desired to have a 57kHz carrier and the bandwidth available spans from 53kHz to 72kHz, the modulated signal can only have 4kHz of bandwidth and will utilize only the frequency band from 53kHz to 41kHz. Wasting space in such a way is not acceptable when dealing with the minimal amount of bandwidth that we have. The single side band technique was therefore chosen for the case of available bandwidth. With this technique, a 15kHz signal can be modulated around a carrier frequency as the upper side-band, while a separate 4kHz signal can be inserted as the lower side-band, therein using all of the available bandwidth.
The implementation of our proof of concept system displayed in Figure 1 can be investigated now that the signal spectrum has been specified. The technical approach towards our system was driven by efforts to reduce cost, development time, and complexity. We therefore designed our system using as large of an amount of off the shelf equipment as available.
Multi-channel emitter
The first block in our system is composed of the multi-channel emitter, this component is an AC-3 compatible surround sound receiver for home theater purposes. Utilizing an off-the-shelf system for the first stage eliminates the requirement for us to perform the AC-3 digital decoding of the 5.1 surround sound channels. The output of the first block is 6 analog audio channels composing the 5.1 channel surround sound audio system.
Base-band Mixer
The 6 analog audio channels taken from the multi-channel emitter are input into the base-band mixer that is responsible for modulating them into the format shown in Figure 2. The mixer will be designed and constructed by our team utilizing either an analog or digital signal processor (DSP) based design. As its advantages, an analog design would reduce construction and design time, while also reducing the system cost. The disadvantages of the analog system would be lower grade synchronization and difficulty in finding parts. The DSP design would be capable of very high performance filtering and modulation, along with providing us with a high degree of control over filter and modulation implementation. The DSP, however, could introduce longer development time and high costs. The development time would increase dramatically as a result of the complex DSP code that would be required. We have thus decided to pursue the analog integration as far as possible, but it appears that we may in the end need to utilize a DSP. The DSP may be used due to the fact that it has been exceptionally difficult to find any analog single side-band modulation chips in the operational frequency range of 0-75kHz.
Modified Commercial FM transmitter
The modified commercial FM transmitter block is exactly what it says, a modified, off-the-shelf FM transmitter. The off-the-shelf transmitter has been specified for two reasons, to reduce system cost and development time, and because our system is being designed such as to be easy to insert into current FM stereo transmission systems. The FM transmitter block is responsible for FM modulating the complex base-band spectrum constructed by the mixer, and transmitting it.
Modified Commercial FM receiver
The FM receiver will be based on an off the shelf commercial FM stereo receiver. The receiver will be modified to output the base-band composite spectrum immediately after FM demodulation. The FM demodulated composite spectrum will then be output to the base-band demodulator.
Base-band Demodulator
The base-band demodulator shall decompose the base band signal into its 5.1-channels utilizing either an analog or DSP implementation. These six signals are fed into separate amplifiers where appropriate volume control is maintained.
Amplifiers / Speakers
The volume-compensated channels are sent to their corresponding amplifiers and speakers, where the audio information is reproduced in its initial format, thereby completing the entire transmission and reception process.
Milestones
Following are the milestones that will be used to evaluate the success of our project through tests at the projects completion.