Deliver it not…
Dear Paul McGoldrick:
Your analysis in the [ June column] was very clear. Free over-the-air broadcasting requires a viable, robust delivery system that viewers can rely on to receive the digital signals. Reliance on the good graces of competing services like cable and satellite only speeds up the eventual demise of free over-the-air TV services.
The one point that you seemed to avoid is the glaring reality that the present fumbling attempts to make 8-VSB work by reducing the data rates to gain robustness will eliminate HDTV as a delivered service. The current secret efforts of NAB and MSTV are equivalent to rearranging the deck chairs on the Titanic after it has hit the iceberg.
Wishful thinking by the NAB and MSTV technical leadership that yet another miracle will save 8-VSB and DTV is wasting time and money. You clearly noted that the European system did work as an over-the-air service. The imposed power level restrictions may have made it difficult to receive, but COFDM modulation proved its worth in the UK.
Isn't it about time we stand back and evaluate the current efforts to make increasingly complex receivers in the vain hope that we will somehow stumble on a solution to the 8-VSB multipath limitation when COFDM is now the default worldwide standard. Who or what are we protecting? Is it egos, personal reputations or patient rights? We certainly are not protecting the American TV viewer.
Vice President-New Technology
Sinclair Broadcast Group
Dear Michael Robin:
As usual, an excellent and informative article! I work primarily as a freelance NLE, and the concept of VU vs. PPM [discussed in your April article] brings up a question about average/peak audio levels and reference signal.
With a VU meter it is simple to set tone to 0 VU and average audio to 0 VU. However, with a PPM, setting tone at 0 displays an “average” level on a PPM at +6 (since peaks are now factored in). Therefore, should I set my level for tone and not worry about riding at +6 (on a PPM) for my audio, or should I first lay down tone at 0 and then set my “average material” audio at 0 on a PPM, giving me an average at -6 VU on a VU meter?
It seems with the latter that I am giving myself 20dB headroom for digital end users, but the former is more correct for the analog world. I want my product to be consistent with standards (if any exist)!
Michael Robin responds:
The concepts of VU and PPM are based on irreconcilable approaches to audio signal level monitoring. The problem gets even more complicated when both types of instruments are used side by side. So, make a choice and stick to it!
It is interesting to note that digital audio equipment manufacturers allow for a headroom of 20dB between the alignment level and the absolute peak level of 0dBFS. This is done to enable the users of VU meters to avoid digital clipping while still maintaining a respectable SNR. This is possible due to the tremendous digital equipment dynamic range of 120dB when using 20 bits per sample. Problems occur with analog background personnel, who will complain that the “loudness” of an audio signal with a 20dB headroom is generally lower than when you reduce the headroom to 6 to 10dB as is customary with analog equipment and VU metering. The latter is necessary with analog equipment due to its poor SNR. I personally would prefer operating with a PPM and a 20dB headroom and, of course, digital equipment. As for standards, it is interesting to note that to the best of my knowledge there are none.
Regards, Michael Robin
The importance of being direct
I am a land surveryor and have been asked to certify to the antennae direction (azimuth) on a newly constructed tower as part of the FCC permitting process. What accuracies are required (i.e. plus or minus two degrees)? Is there a standard statement or certification that addresses this issue?
James M. Overfelt, RLS
Barge Waggoner Sumner & Cannon
TV antennas can be quite directional, depending on frequency, design and mounting configuration. In the case of a top-mount omnidirectional antenna, there really isn't an azimuth as such; radiation is pretty equal in all directions.
A side-mounted antenna on a high channel would certainly be directional, in part because of the effect the tower has on the antenna's pattern.
Antennas are typically modeled at the factory on either full scale or fractional scale models of towers, and patterns are plotted for the customer. Using a chart and the actual mounting configuration, you would be able to project the pattern out to the horizon.