In the late 1930s PPMs were considered for use in the US, but rejected in favour of a 'Standard Volume Indicator' (VU meter) on grounds of cost. Joint research by CBS, NBC and Bell Labs found that using an experimental design of PPM (with a relatively long integration time of 25 ms) in the control of programme levels gave only a 1 dB advantage over the VU meter, in terms of average output level for a given amount of distortion. It was felt that this was too small to justify the much greater expense. It was also found that VU meters gave more consistent readings than PPMs when comparing programme levels at the sending and receiving end of long lines subject to group delay, which altered the waveform. This finding has been disputed by others.