Dear Treggers, I wonder if any modem and DAA designers can give us some advice. We are developing a DAA for a V.34 modem. Our design is based on a "dry" Midcom 671-8056 transformer, connected to the line via a 2.2 uF capacitance . The modem itself is a proprietary design based on a DSP and analog front-end (A/D and D/A sigma-delta converters).
The modem DSP software designer is asking us to improve the transhybrid loss (THL) characteristic by making it flatter against frequency. He is complaining that the THL of our design droops for low frequencies of around 150 to 400 Hertz, compared with the value for higher frequencies, when the DAA is connected to a line comprising 600 ohms + 2.2 uF. Measurements indicate THL values of approx. 19, 22 and 28 dB, at 100, 300 and 1000 Hertz, respectively. We have explained that near-end DAA echo-cancelation necessarily represents a compromise, since line impedances vary considerably, and we designed for an "average" sort of line. According to our simulations, we can expect measured THL responses to be even less steady than the results measured for a 600 ohm + 2.2 uF line, when the circuit is tested against a full range of line impedances. We have stated that we could improve the response for a specific line type to the detriment of response against other line types, but that such a step would not be beneficial. Since the software designer's complaint refers to low frequencies on the order of 200 Hertz, it might be possible to increase the insertion loss in both the transmission and reception path at these low frequencies. However, I am concerned that this might adversely affect modem performance in other respects (due to increased group delay, or some other factor). Naturally, we are keen to keep the design straightforward and avoid any exotic adaptive hardware, etc., which nobody else seems to require. It seems to us that other modem software designers have reconciled themselves to a far-from-ideal THL response from simple, inexpensive DAA designs, and manage to cope satisfactorily. With reference to the above, could anyone tell us the following: 1. Is the described THL response against frequency for our design typical of DAA designs for V.34, or is our design particularly bad?! 2. What kind of flatness against frequency should we be aiming for, based on the accepted state-of-the-art reached by V.34 software, and which other V.34 DSP software has to cope with? 3. What "average" line impedance do you recommend us to choose, against which we should design for best THL response? I have seen figures of 350 ohms in series with a parallel combination of 1000 ohms and 0.21 uF. 3. Can anyone confirm or dispel our worries about the possible adverse effects of deliberately increasing the insertion loss for low frequencies (below around 400 Hertz), in order to minimize THL degradation at these frequencies? Any advice on the above will be enormously appreciated, and will make us feel quite sure of ourselves when talking to the software designer - or cause us to try and improve our design - as the case may be. Best regards, David Drori ----------------------------------------------------------- Novarex Enterprises Ltd., POB 2833, 306-6, Mevo Hapartisanim Street, Jerusalem 91028, Israel. Tel: +972 2 581 0995 Fax: +972 2 581 3750 -----------------------------------------------------------
