An isochronous output data stream format in which a single n bit word is transmitted as m multiple synchronous data streams where the length of the word transmitted in each data stream is n/m, the output data stream containing multiple isochronous different data rate data streams and one or more asynchronous data stream or streams. One bit from each of the m multiple synchronous data streams are combined and transmitted as a single signal at one of 2m analog levels. Data which is it desired to have a lower error rate than other data is transmitted in the synchronous data stream corresponding to the most significant bit associated with the 2m analog levels. Calibration data is transmitted in the synchronous data stream corresponding to the most significant bit associated with the 2m analog levels, the calibration data also being transmitted distributed over each of the m synchronous data streams.