JVC Professional APRIL 2007 NAB-2007 The ProHD Report
Copyright 2007 JVC Professional Products Company All rights reserved Page 17 of 43
JVC ProHD Camcorder with
BMS cost effective
Microwave Camera-back
But new digital microwave technology often utilizes COFDM multi-carrier
transmitter, which enables non-line-of-sight links (multi-path) in metro areas and in
special events coverage, coupled with a QAM modulation scheme. Cable television
is using 64-QAM and 256-QAM on single carriers to pack hundreds of SD TV
channels (and some HD channels) on one coaxial cable. The higher the QAM
number, the higher the bitrate transmission capability over a given bandwidth, but, as
the QAM number is increased, the receiver input requires an ever stronger signal
(higher SNR) to reliably decode the modulation. It is a trade-off between higher
bitrates and shorter distances in the HD ENG microwave world. 256-QAM is
easily done through a fiber or coaxial cable, as it’s a controlled wired transmission
medium, but 256-QAM is very difficult in HD ENG wireless applications, as
microwave camera-backs don’t have enough TX power and need to use omni-
directional whip antennas for the camera-back TX unit as well as for the RX unit (a
requirement for dynamic multi-path “roving” performance), generally resulting in
unreliable link for 256-QAM.
Fig. 4. JVC’s ProHD ENG
camcorder fitted with BMS 2GHz
microwave camera-back unit.
The BMS camera-back accepts
the compressed MPEG-2 TS
(Transport Stream) of 20Mbps,
modulates 16-QAM and transmits
COFDM over 8MHz bandwidth for
roving robustness in HD ENG,
sports and EFP applications. This
space saving ProHD package
offers excellent weight
distribution both for shoulder-
use and for hand-held.
From 18MHz channels down to 12MHz? The 2GHz BAS relocation reduces
channel bandwidth to 12MHz. Can 12MHz do the job? For SD links, 12MHz is
ample bandwidth. There is even industry talk of being able to provide reliable two
channels of 6MHz each within the 12MHz channel for SD service. But with
COFDM, you run into a problem called “spectral regrowth” of the large number of
carriers within a single channel with COFDM transmission, causing adjacent channel
interference due to the out-of-channel spectral regrowth. The solution is to limit the
actual COFDM bandwidth to 8MHz within the 12MHz channel, providing for guard
bands of 2MHz on each side. Thus the effective COFDM/QAM channel bandwidth
becomes only 8MHz in the relocated 2GHz band (referred to as 8MHz pedestal),
with the following performance limitations: