This work studies the asynchronous sequential symbol synchronizers based on pulse comparison by both transitions at half bit rate. Their performance will be compared with the reference asynchronous symbol synchronizers based on pulse comparison by both transitions at bit rate. For the reference and proposal variants, we consider two versions which are the manual (m) and the automatic (a). The objective is to study the four asynchronous synchronizers and evaluate their output jitter UIRMS (Unit Interval Root Mean Square) versus input SNR (Signal Noise Ratio).