It has been argued by several in this group that the first part of a digital mode transmission may be deleted by faulty transmit hardware without any problems in the reception on the other end. In other words, the first part of a transmission may be thrown away or discarded, and the message will still get through.
One reason they give is that some modes are designed with preambles or first parts of the transmission as simply a synchronization for the signal, without any actual message content. They also say that the FEC (Forward Error Correction) or ARQ, or other repeat request parts of the mode will somehow mysteriously put the lost part of the message back together :) However, in my small scope of experience, as one who has been using and testing various ARQ, handshaking, and other soundcard programs for only 8 years, I can tell you that in most ARQ modes and handshaking modes, every part of the mode's transmission is being transmitted for a good reason... and the first part of each transmission is intended to be transmitted and received. It is not something you can willy-nilly throw away in the hardware, and expect the mode to perform correctly. In some modes, the first part of the transmission is relied upon to lock the decoder's synchronization so the the rest of the message can be decoded reliably. If the initial part of that transmission is deleted by the sender's hardware, then the decoder on the other end of the radio path will not be able to lock on to the signal as well, and this will cause random errors. In the case of modes that have repetitive or redundant transmission built into them, (in other words, they transmit the same thing several times, to increase the probability that the content will get through) the intention of the redundancy is to overcome the noise or interference of the radio path. If you delete one or more of the repetitions of the transmission, by using a faulty VOX interface or faulty hardware for whatever reason, you are defeating the redundant part of the signal, and thus, your dependability of the message will certainly suffer. I've personally observed this happening on the air. Often, the operators have no idea that it is even happening. And often, the operator isn't aware that their own hardware is causing the problem. They blame the mode, the software, the propagation, or even the other operator. But they don't want to consider for a moment that their own choice of hardware, that they paid good money for, may be at fault :) Sometimes, because they don't see the problem happening in one or more QSOs, they assume they have no problem. The redundancy has corrected the errors. But, reliability problems may show up in other instances when conditions are not quite so good, or if the station on the other end of the QSO is not optimized. These conditions are not something that the operator can control. But, good hardware to offset these problems is certainly within the realm of what the operator can do. There are some who have suggested that modes can be designed that can compensate for such initial deletion of the transmission. But, the fact is, that there are many existing modes that don't, and they will not be changed. In fact, especially for the faster ARQ systems, it makes no sense, because, the result certainly would be to slow down the throughput of the system. The trend now in both ham radio and commercial radio, is toward Software Defined Radios, and toward very rapid DSP systems. This enables even faster handshaking methods for digital communications than ever before thought possible in economical hardware. Thus, the trend in new modes of the future can be open to even tighter timing in ARQ handshaking and time multiplexing. Often, the discussion of this group revolves around the fads in software. But, I believe that the software is only as good as what the hardware that sends and receives it can provide. 73 Bonnie VR2/KQ6XA "Caveat Emptor - Etymology: 1523 Latin, let the buyer beware. A principle in commerce: Without a warranty the buyer takes the risk"
