Subject: TDT system 2 users From: "McAnally, Ken" <Ken.McAnally(at)DSTO.DEFENCE.GOV.AU> Date: Mon, 13 Sep 2004 17:06:10 +1000Dear List, Following is a discussion for users of TDT system 2. My intention is not to make comment on the hardware, but rather to pass on information that is not easily found in the TDT documentation. While piloting a new experiment it became evident that the TDT system 2 hardware may produce substantial distortion using typical parameters for stimulus generation (e.g. those specified in the TDT tutorials). For example, generation of a 16 kHz tone using our PD1 using a sample rate of 50 kHz and the full range of the D/A converter resulted in large distortion products. (Distortion around 2 kHz was only 24 dB below the desired signal, which has obvious implications for high frequency audiometry.) These are not aliased frequencies and are not removed by anti-aliasing filters. TDT technical support were helpful in diagnosing the problem as a slew rate limitation in the D/A converters and suggested that signals should be generated either using a high sample rate, or at a low output level (e.g., perhaps a quarter of the full D/A range, i.e. using 14 of the 16 bits). This solved the problem, but reinforces the need to check the stimuli carefully, even when using dedicated hardwa! re for auditory research. I could not find any information on this issue in the TDT documentation or website except for a discussion of the DA3 where slew-rate is selectable. http://www.tdt.com/T2Support/technical_notes/tn0121.pdf I do not know how the system 3 hardware performs in this regard. cheers Ken McAnally