NOW! 12-13 - Telos

More documents

Recommendations

Info

OMNIA | FM-STEREO TRANSMISSION | TECHNOLOGY ARTICLE 56 SINGLE SIDEBAND SUPPRESSED CARRIER (SSBSC) Figure-13, Peak Level, 15kHz, L-Ch Notice the reduction in utilized RF spectrum. The signal shown in Figure–12 will pass through narrow cavities, combiners, and mal-adjusted antennas with better stereo performance than the broader signal shown in Figure–10. Additionally, there are less sideband pairs of the carrier signal. Less sideband pairs equates to less signals, which can be inter- fered with during instances of multipath. In the United States, FM channel spacing is maintained at 200kHz. This is a bit of a luxury compared to the rest of the world where channel spacing is usually 100kHz. Consider the probability of less channel-to-channel interference when SSBSC transmission could be used. This would appear to be an improved alternative to the ITU BS-412 MPX power regulation, which is now in force in some European countries. Another mentioned beneit SSB brings to the transmission method is added spectral protection to RBDS, SCA, and HD Radio services as observed in Figure–14. Single channel only pink noise is used to generate the baseband signal with SSBSC modulation. Notice the extremely wide guard band that exists between 38kHz and where the irst SCA carrier would appear at 57kHz. The reduction in cross-talk to ancillary services is exceptional! Figure-14, Reduction in Cross-Talk to Ancillary Services. SSBSC AND MODULATION PEAK CONTROL Implementing SSB can be accomplished using numerous tech- niques. The most common method is through use of the Hilbert function, where a 90 degree broadband phase shift is used to cancel the undesired sideband. It can also be achieved using a Weaver modulator, or a low pass ilter set to critically limit the desired passband, and the undesired sideband is removed through iltering. All of these methods provide satisfactory SSB operation, but there is a critical element that must be con- sidered…peak control of the overall MPX signal. In each of the afore-mentioned SSB methods, there will be alteration to the phase relationship of the sideband signal. This alone will generate overshoot to MPX encoded signal [3]. It is paramount that SSB modulation must not add any overshoot to the signal, and it must not add any unwanted non-linear components, in the form of audible overshoot peak limited harmonic content, i.e. clipping by-products. The sonic performance of the SSBSC modulator must perform sonically, exactly the same as the DSBSC counterpart. Switching from DSBSC mode to SSBSC should not change the resulting sound in stereo separation, audio quality, and peak control. Theory indicates that a 90 degree phase network, in the form of a Hilbert ilter will cause overshoot to a square wave. What’s interesting is by adding a second Hilbert function, the overshoot is removed, and the square wave is recovered. Use of the double Hilbert function has been referred to as the “Dilbert” function [4]. An example of this is provided in Figure-15. Figure-15, Hilbert Affect on Square Waves Normalized Amplitude 2 1 0 -1 -2 -3 0 0.001 0.002 0.003 0.004 Time (Seconds) 0.005 0.006 0.007 400 Hz Band Limited Squarewave Hilbert Transform of Squarewave Dilbert Transform of Squarewave Just as an audio processor is known to employ non-audible methods to eliminate peak overshoots in the required 15kHz low pass ilters, there are non-audible algorithms employed in the SSBSC generator which insures that any overshoots are eliminated, and done so without any sonic change or affect to audio quality. Figure-16 is a screen capture from a digital oscil- loscope that was measuring real world MPX signal at the output of an audio processor. Figure-17 is the spectral reproduction of the same signal. Note exceptional peak control along with the well maintained spectrum around the 19kHz pilot, and the sharp drop off after 38kHz.
Figure-16, MPX Peak Control, SSBSC Figure-17, MPX Spectra, SSBSC TECHTALK BLOG TELOSALLIANCE.COM/BLOG SSBSC AND DECODED SIGNAL-TO-NOISE Another known challenge for the system is the compromised signal-to-noise (SNR) level when broadcasting stereo. FM transmission noise will rise in a triangular fashion at 6dB per octave over the channel’s passband range of 0Hz-99kHz. This is the product of the modulation/demodulation process. The use of preempha- sis in transmission, along with complementary deemphasis in demodulation, improves the high frequency noise response. It has been generally accepted that FM-Stereo suffers a 23dB overall noise penalty compared to monophonic broadcasting. This is due to the rising noise loor over the subcarrier range of 23kHz – 53kHz, as compared to the SNR over the range of 0Hz – 15kHz, which is used for mono. Figure–18 is an illustration of the com- posite baseband signal, and it shows the 6dB/octave noise loor slope of an FM channel, as it would appear at the output of an IF section in a receiver. Figure-18, FM System Noise Plot BROCHURES OMNIAAUDIO.COM/BROCHURES SOFTWARE UPDATES OMNIAAUDIO.COM/SOFTWARE It has been theoretically calculated [5], and technically demon- strated, there is roughly a 4dB broadband improvement in recovered signal-to-noise performance of the SSBSC transmit/receive function, as compared to the conventional DSB transmit/receive iteration. When transmitting SSBSC, and decoding only the lower sideband spectra (23kHz – 38kHz), an interesting event occurs. Stereophonic noise is about 10dB better for decoded 15kHz. This is due to the frequency inversion of the lower sideband. The triangular noise is lower at 23kHz, where 15kHz resides in the lower sideband region of the L-R signal, as compared to lower frequencies, which are located near 38kHz and triangular noise is greater. Figure-19 is the recovered noise loor of a DSBSC transmission/ reception. Compare the amplitude of the noise loor at 15kHz in this igure with that of Figure-20, which is the recovered noise loor of a SSBSC transmit/receive system. Figure-19, Recovered Noise, DSB Figure-20, Recovered Noise, SSB Consider the annoying hiss a listener hears at the output of the FM receiver. The predominant range of audible hiss is the high frequencies. As observed in Figure-18, there’s an improvement of 10dB in signal-to-noise in the audible hiss range. Hopefully, this might encourage receiver manufacturers to consider adding SSB decoding into conventional receivers. The above test results were realized using a SSBSC stereo decoder designed and imple- mented, real time, in MatLab by the author. FIND A DEALER OMNIAAUDIO.COM/BUY AUDIO PROCESSING | FM | FM+HD | AM | MULTICASTING | CODED AUDIO | STUDIO APPLICATIONS OMNIAAUDIO.COM 57
Page 2 and 3:
TELOS ALLIANCE | COMPANY HISTORY 2
Page 4 and 5:
TELOS ALLIANCE | TABLE OF CONTENTS
Page 7 and 8: TECHTALK BLOG TELOSALLIANCE.COM/BLO
Page 11: TECHTALK BLOG TELOSALLIANCE.COM/BLO
Page 14 and 15: TELOS | BROADCAST PHONE SYSTEMS 14
Page 16 and 17: TELOS | BROADCAST PHONE SYSTEMS 16
Page 35: TECHTALK BLOG TELOSALLIANCE.COM/BLO
Page 38 and 39: OMNIA | RADIO PROCESSING 38 OMNIA.1
Page 40 and 41: OMNIA | RADIO PROCESSING 40 OMNIA.9
Page 42 and 43: OMNIA | RADIO PROCESSING 42 ONE FM
Page 44 and 45: OMNIA | RADIO PROCESSING | APPLICAT
Page 46 and 47: OMNIA | CODED AUDIO 46 OMNIA A/XE T
Page 48 and 49: OMNIA | CODED AUDIO 48 OMNIA.8x EIG
Page 50 and 51: OMNIA | CODED AUDIO | APPLICATION A
Page 52 and 53: OMNIA | FM-STEREO TRANSMISSION | TE
Page 65 and 66: + 4-Fader modules accommodate any s
Page 77 and 78: + + + TECHTALK BLOG TELOSALLIANCE.C
Page 81 and 82: Silky-smooth, side-loading 100mm. c
Page 96 and 97: AXIA | NETWORKING | TECHNOLOGY ARTI
Page 105 and 106: LINEAR ACOUSTIC Viewers are listeni
Page 107 and 108:
TECHTALK BLOG TELOSALLIANCE.COM/BLO
Page 109 and 110:
Page 111 and 112:
Page 113 and 114:
Page 115 and 116:
Page 117 and 118:
Page 119 and 120:
Page 121 and 122:
Page 123 and 124:
show all

NOW! 12-13 - Telos

You also want an ePaper? Increase the reach of your titles

Delete template?

Save as template?